to assist in the description of these components , the following coordinate terms are used . fig1 depicts an x - y - z cartesian coordinate system , with the container system primarily lying in the x - y plane . as described herein , terms such as “ height ” refer to distance in the z - direction , and “ higher / upward ” and “ lower / downward ” refer to the positive and negative z - direction , respectively . similarly , terms such as “ outward ”, “ inward ”, and “ radial ” generally refer to directions in the x - y plane directed generally away from or toward the center of the container system , although not necessarily directly from or toward the center . similarly , terms such as “ lateral ” will refer to the y - direction and “ longitudinal ” will refer to the x - direction . a detailed description of a preferred embodiment of the container system , and its associated method of use , now follows . fig1 - 3 illustrate an embodiment container system 1 including a base 30 with an upwardly - opening cavity 44 for holding goods such as food , and a lid 10 that closes the top of the cavity . the base 30 and lid 10 are preferably formed of a single sheet of plastic that has been thermoformed as by vacuum forming , transfer molding or blow molding . it is even possible to injection mold the container to form a container comprising a plastic sheet . the base 30 and lid 10 can include center portions 11 , 31 surrounded by second , third , and fourth sides 13 - 15 , 33 - 35 that are securely held to each other so the sides cannot be separated until after the first sides 12 , 32 have been separated . the first sides 12 , 32 of the base 30 and lid 10 are initially fixed together along a pivoting join line 72 formed in an integral portion of the plastic sheet , that can be easily torn . along the first sides 12 , 32 , the lid 10 has a lid tab 20 that projects radially outward with respect to the container system &# 39 ; s vertical axis . as depicted , the container system 1 can have a generally rectangular shape in the x - y plane , with the lid tab 20 and the pivoting join line 72 on a shorter edge of the container . however , in other embodiments the container system 1 can have other geometric properties , such as being generally rounded . in some embodiments , the lid tab 20 can be no greater than 0 . 14 inches thick . fig2 and 3 show the container system 1 as it can be supplied to a store . the container system 1 can initially be in an open position , such that a store clerk can load goods into the container system and initially close the container . at this point the first sides 12 , 32 of the base 30 and lid 10 are joined along the pivoting join line 72 , but the second sides 13 , 33 are separated . the base 30 and lid 10 can have the form of thin plastic , or plastic sheeting , and in a preferred embodiment are parts of the same sheet of plastic ( which may have portions of different thickness or other material properties ). the join line 72 can be formed in an integral sheet of plastic . fig3 shows the container as it can be initially formed from a single sheet of thermoformed plastic which has been thermoformed to the illustrated shape , and with slits cut along the join line 72 . the lid can be folded , or bent 180 degrees from fig3 at a hinge 70 to achieve the initially closed position of fig1 . fig2 a shows the first sides 12 , 32 of the base 30 and lid 10 , showing that the join line 72 includes opposite join line parts 72 a and 72 b that lie on opposite sides of the lid tab 20 . along each join line part 72 a , 72 b there are three join lengths 74 and an interrupted slit 76 with slit parts . in some embodiments , the join lengths 74 can each have a length of no more than 0 . 25 inch . leading to each join line part 72 a , 72 b is an entrance slit 78 with an end width of the same order of magnitude as the slit 76 , that leads from the tab 20 to the join line parts , to concentrate separation forces along the join line parts . in addition , the plastic sheet can be creased ( by forming a sharp bend such as a bend having a radius of curvature no more than the thickness of the two sheet portions being bent ) to weaken it along the join line . it can be seen in fig2 a that the lid tab 20 has a radially outward extension 21 that projects beyond the pivoting join line 72 longitudinally . the base 30 can have a cutout 42 initially lying around the lid extension 21 ( when the lid is pivoted 180 degrees from its closed position to the open position in which it is manufactured ). as depicted , the cutout 42 can have two longitudinal walls and one lateral wall , forming a generally rectangular structure , but other shapes are possible such as a rounded cutout , a semi - circular cutout , or a generally polygonal cutout . the lid tab 20 can be formed by the cutout 42 which can extend continuously to leave a lid tab extension 21 with a generally rounded rectangular shape . the cutout 42 can leave clearance between the longitudinal and lateral cutout walls for the rotation of the lid tab 20 with no scrap plastic sheet to throw away during production of the lid tab extension 21 . the tab extension 21 can end in an entrance slit 78 leading to the join line parts 72 a , 72 b . in other embodiments , the base 30 can also have a tab with a similar extension . the lid can then form a cutout around the base extension ( further details depicted in u . s . patent application publication no . 2006 / 0289549 , which is incorporated herein by reference in its entirety ). these various structures provide longer tabs that can be more easily grasped and cutouts under or over the tabs . fig2 a also shows that the first side 32 of the base forms a join rim part 80 that can be flat and lie facewise adjacent to the lid tab 20 when the lid 10 is closed on the base 30 . the rim part 80 of the base can also have bumps 40 that assure slight separation between the lid tab 20 and the join rim part 80 after the join line is torn . fig2 b depicts another embodiment container system , similar to that depicted in fig2 a with similar features denoted with corresponding numerals . as depicted , the container system can include two join lines 72 ′, 72 c ′, each with entrance portions 78 ′. accordingly , the first join line 72 ′ can separate upon activation of the lid tab 20 ′. however , in the embodiment of fig2 b the second join line 72 c ′ can remain intact after activation of the lid tab 20 ′, retaining a removable strap 82 ′ connecting the lid tab 20 ′ to the lid 10 ′. accordingly , this strap 82 ′ ( and the lid tab 20 ′) can be removed at a later time , if desired , by pulling upon an edge of the strap 82 ′ to tear the second join line 72 c ′. this may be desirable , for example , if a consumer has already purchased the container system and its contents , and no longer desires the hangability provided by the tab 20 ′. once the tab 20 ′ and the strap 82 ′ have been removed , the remaining portions of the join rim part 80 ′ on the base 30 ′ surrounding the cutout 42 ′ can extend beyond the lid 10 ′, and thus function as tabs to facilitate opening and closing of the container system . a clerk at a store can receive a stack of containers in the configuration shown in fig3 , with the first sides 12 , 32 of the base 30 and lid 10 joined and with the second sides 13 , 33 widely separated . the clerk can first load goods such as food into the cavity 44 of the base , and then pivot the lid 10 so its second side 13 lies over the base second side 33 . the clerk then can force the lid second side 13 downward to lock , or latch it to the base second side 33 , to place the container in an initially closed configuration . fig1 shows the container in such an initially closed configuration . fig2 c shows one embodiment interlocking structure on the third sides 14 , 34 of the base 30 and lid 10 after the lid has been initially closed on the base . as depicted , the third side 34 of the base 30 has a free rim 50 with a raised ridge 54 . at an inner side of the ridge 54 , an undercut 58 can be formed with an upper limit surface 58 and a lower limit surface 59 . the lower limit surface 59 can extend to two or more seal walls 60 , 62 which can be separated by a seal ridge 64 . from the lower seal wall 76 , the side walls 36 of the base 30 can extend toward the center portion 31 . these features can interact with corresponding features on the lid 10 . as depicted in fig2 c , the lid 10 can include a free rim 52 . the free rim 52 can extend inward , and two seal walls 61 , 63 can extend downward from the free rim , separated by a seal ridge 65 approximately matching the seal ridge 64 on the base 30 . in operation , the lid 10 and the base 30 can interengage when pressed together as depicted . when pressed together , the free rim 52 on the lid 10 and / or the undercut 56 on the base 30 can deflect to allow the free rim to enter the undercut 58 . the undercut 58 can then remain between the upper and lower limit surfaces 58 , 59 of the base 30 . in this position , the lid 10 and the base 30 can thus be held in a facing relationship . further , with the free rim 52 within the undercut 58 , the upper and lower seal walls 60 - 63 can engage each other to form a substantial seal . as depicted , the form of the seal walls 60 - 63 and the seal ridges 64 , 65 can create two sealing regions . however , in other embodiments one or more than two sealing regions can be formed . further , although this interengagement is depicted as being on the third sides 14 , 34 , such sealing features can also extend about the second and fourth sides 12 , 14 , 32 , 34 , and additionally onto portions of the first sides 12 , 32 . further details of such sealing features can be found , for reference , in u . s . patent application publication no . 2007 / 0138180 , which is incorporated herein in its entirety . advantageously , the interaction between the free rim 52 of the lid 10 and the undercut 56 of the base 30 can reversibly hold the lid and base together . as depicted in fig1 , 2 c , the perimeter of the lid 10 can be substantially contained by the perimeter of the base 30 , preventing lateral and longitudinal movement of the lid 10 when in the closed position . further , the free rim 52 of the lid 10 , as discussed above , can be restrained vertically be the undercut 56 . accordingly , the lid 10 in the illustrated embodiment cannot be removed from the base 30 absent some deflection in the lid or base . further , in some embodiments the interaction of the seal walls 60 - 63 can form a substantially liquid - tight seal within the container system 1 . accordingly , when a clerk presses down the lid on the base during initial closing of the container , the lid can move down and deflect ( as can the base along with or instead of the lid ) until the free rim 52 of the lid 10 enters the undercut 56 . the limit surfaces 58 , 59 can then restrain the free rim 52 in its final closed position , shown in fig2 c . this motion into the closed position can occur as the lid 10 rotates about the hinge 70 relative to the base 30 . although the hinge 70 , as depicted and discussed herein includes a join line 72 that is perforated after formation , in other embodiments the hinge can be formed in other ways such as by sonic welding , glue , or the like . in further embodiments , the join line 72 can be unperforated . for example , to provide sufficient flexibility without perforations , in some embodiments the join line 72 can comprise a material thickness of no greater than 0 . 06 inches . as depicted , there are no tabs or other projections along the lid second , third , or fourth sides 13 - 15 that can be grasped to pull it up . accordingly , after a clerk initially closes the lid 10 on the base 30 , a customer cannot easily open the container without tearing the plastic sheet along the pivoting join line 72 , as by forcefully lifting the lid tab 20 while holding down the base ( for example holding down the base by holding the ridge 54 along the base rim 38 ). when the customer tears the plastic sheet along the join line 72 , he / she thereafter can easily close the container , and can reopen the container by lifting the lift tab 20 with a small force ( e . g . less than one pound in some embodiments ) as the join line has already been torn . further , the base 30 can include bumps 40 near the join line 72 such that , after the pivoting join line 72 has been torn , the lid 10 and base 30 can be separated slightly while in the closed position to indicate to viewers that the container may have previously been opened . in some embodiments the lid 10 can only be restrained on the second side 13 . thus , when the first side of the container is opened by lifting the lid tab , the second side of the lid can slide slightly towards the first side so the lid free rim 52 does not lie under the base shoulder and therefore the lid can be easily completely separated from the base . advantageously , after the clerk has closed the container system 1 , the container system 1 can have a hanger hole 24 on the lid tab 20 , extending outward from the container system . the container system 1 can thus hang on a standard retail hanger , or any other suitable hanging apparatus . as the entire hanging system can be created from a single sheet , this design advantageously increases the structural integrity of the container system 1 . further , provision of the tab 20 along with the cutout 42 simplifies the production of the container system 1 by reducing material that must be removed from the molded material . integrating the tab 20 and the hanger hole 24 further reduces the number of protrusions from the container system 1 , simplifying the design . providing even further advantages , the container system 1 can have elements indicating freshness , and be resealable after an initial opening . fig4 a - c depict another embodiment container system 201 . as depicted , the container system 201 can include a base 230 and a lid 210 , with a base tab 220 on the base and a cutout 242 . the base tab 220 can have a circular hanger hole 224 . as further depicted , the perimeter sealing between the lid 210 and the base 230 can be substantially the reverse of the sealing between the lid 10 and the base 30 in the embodiment of fig1 - 3 . the lid 210 can include a ridge 254 with an undercut 256 , and the base 230 can include a free rim ( not shown ) extending into the undercut . as depicted in fig4 c , once a perforated join line 272 has been broken , it can be visible to a viewer , with the separation between the base tab 220 and the join rim part 280 on the lid 210 amplified by the bumps 240 fig5 a , 5 b depict another embodiment container system 301 . this embodiment can be substantially similar to the embodiment container system 201 in fig4 a - c . however , the container system 301 in fig5 a - b can include a hanger hole 324 with a side opening 324 a allowing the container system to engage a hanger from the side . alternatively , the previous embodiments typically enter a hanger from an end . fig6 , 7 , 7 a depict another embodiment container system 401 . the container system 401 can be substantially similar to the container systems described in the previous figures , including a lid 410 and a base 430 . as depicted , the lid 410 includes a cavity 444 , and the base includes projected walls 448 . the projected walls 448 can advantageously be shaped to match the shape of an intended product held by the container system 401 , and the side walls 416 on the lid 410 can be shaped to receive the corner walls 448 . as further depicted , both the lid 410 and the base 430 can include hanger holes 424 on their respective tabs 420 , 446 . however , the entrance slit 478 can be extended and arculate , as compared to the previous embodiments . accordingly , as depicted the entrance slits 478 can create side tabs 420 a on the lid 410 that correspond with cutouts 442 on the base 430 and the outer edges of the container system 401 . in opening the container system 401 , a customer can pull the side tabs 420 a to tear the join line 472 . then , the container system 1 can be easily opened by separating the tabs 420 , 446 . fig8 - 14 depict another embodiment container system 501 that can be substantially similar to the previous embodiments . the container system 501 can comprise a dual - compartment design , including first and second lids 510 a , 510 b and first and second bases 530 a , 530 b . as shown , the bases 530 a , 530 b can be adjacent and joined , with hinges 570 a , 570 b on the sides . the first lid 501 a can include a first lid tab 520 a that can easily enter a base tab slot 545 when the first compartment is closed . the first compartment can then be held closed by a perimeter seal similar to those discussed above . the first compartment can be opened by pulling on the first lid tab 520 a , which can be inaccessible from outside the container system 501 , as further described below . still referring to the embodiments of fig8 - 12 , fig1 depicts the container system 501 near a half - closed position with the first compartment near a closed position and the second compartment in an open position . as shown in fig1 , the first lid tab 520 a can be received entirely by the tab slot 545 . next , the second compartment can be closed , as the second lid 510 b joins the second base 530 b , entering the position depicted in fig8 , 9 , 12 , 13 . as best depicted in fig1 , 14 , the second lid 510 b can include a lid tab 520 b with an inwardly - facing circular groove 526 b surrounding an inwardly - extending circular protrusion 526 a . further , the lid tab 520 b on the second lid 510 b can include a free end 552 extending substantially around the lid tab and including a c - shaped ridge . the second base 530 b can include a base tab 546 connected to a circular protrusion 547 within a recessed generally circular groove 549 . the circular protrusion 547 can include a free end with a c - shaped ridge , as shown . as depicted in fig1 - 14 the circular protrusion 547 on the second base 530 b can engage the circular protrusion 526 a on the second lid 510 b , as can the join rim part 580 with the free rim 552 on the lid and analogous portions on the interior from the protrusions ( on the left side of fig1 ). advantageously , these engagements can create a seal that compliments sealing features discussed above as extending around the remainder of the perimeter of the container system 501 . an alternative embodiment is depicted in fig1 a , with similar features to those of fig1 denoted with a corresponding numeral . as shown , the second base 530 b can include a circular protrusion 547 ′ and a circular groove 549 ′. the circular protrusion 547 ′ on the second base 530 b can generally match the circular groove 526 b ′ on the second lid 510 b , creating a substantial seal between the two . further , the circular groove 549 ′ on the second base 530 b can generally match the free rim 552 ′ of the second lid 510 b , substantially creating a second sealing portion . upon closing the second compartment of the container system 501 , a hanger hole 524 can be formed , passing through the circular protrusions and groove 526 a , 526 b , 547 on the second lid and base 510 b , 530 b . additionally , as best depicted in fig1 , closing the second compartment can conceal the first lid tab 520 a , preventing opening of the first compartment . in both embodiments ( fig1 , 14 a ), the free rim 552 of the second lid 510 b can engage an undercut 556 on the base tab 546 . referring back to fig1 , the base tab 546 can include two join lines 527 a , 527 b extending inward toward the circular protrusion 547 . to release the second lid 510 b from the second base 530 b a customer can pull the base tab 546 , simultaneously causing the join lines 572 a , 572 b to break and the free rim 552 to release from the undercut 546 , as in the previously described embodiments . once the second compartment has opened , the first lid tab 520 a is accessible and can be lifted to open the first compartment . fig1 - 18a depict another embodiment container system 601 , substantially similar to the embodiments discussed above , especially the embodiment container system 1 depicted in fig1 - 3 . as depicted , the container system 601 can be another dual compartment container system . as shown , the two bases 630 can be connected by a compartment hinge 690 on a lateral side , and to their respective lids 610 on a longitudinal side . as depicted , the lids 610 are not joined , allowing them to be opened and closed independently . further , one of the two bases 630 can include a base tab 646 with a hanger hole . although the container system 601 is primarily depicted as having only one base tab 646 at a longitudinal end , additional base tabs can be provided , as shown in phantom on a second , complimentary longitudinal end and / or on a lateral end . the lids 610 can each include an s curve 628 separating two portions of the lid 610 at different heights , creating an s - shaped ridge . as further depicted in fig1 a , the s curves 628 can have undercuts 629 which can form an interfering snap - fit interconnection when the lids 610 are face - to - face , as discussed below . as depicted in fig1 - 18a , the container system 601 can be closed into a fully compact position . in fig1 , the container system 601 is in a fully open position , from which it can be loaded with product . as depicted in fig1 , the lids 610 can be closed independently . once both lids 610 are in the closed position , the two bases 630 can rotate along the compartment hinge 690 , as shown in fig1 , to a fully compact position depicted in fig1 . advantageously , the s curves 628 on the lids 610 can interact in the contact position to further stabilize the container system 601 , as their respective ridges and undercuts 629 snap - fit together . providing further advantage , the compact container system 601 can be hung on either side , through the base tab 646 or the now - aligned lid tabs 620 . alternatively , the container system 601 can be hung in a non - compact position , with the two lid tabs 620 acting separately to hang the container , spreading the container &# 39 ; s weight . the container can be formed from a wide variety of polymers , including polystyrene , polypropylene , polyethylene terephthate , etc . instead of a container of primarily rectangular shape as illustrated in a plan view , the container may be circular , elliptical , or of any reasonable polygonal shape . the base ( and cover ) can have walls forming compartments , and the container can include a middle tray . thus , the invention provides a container comprising a base and lid that can be constructed at low cost , that can be shipped to a store with its cavity open to enable easy loading of food or other goods by a store clerk , and that can be easily closed by the store clerk . the base and lid are preferably formed of a single sheet of plastic , and are joined by a join line that can be torn . after initial closing by the store clerk , the container strongly resists initial opening , to greatly discourage customers from secretly opening the container as to taste food in it . instead , initial opening requires forceful opening by lifting a tab on the lid to tear the plastic sheet along the join line . once the container has been initially opened , the fact that it has been initially opened is evident to anyone who looks at the container , which assures buyers that the container is intact . the container thereafter can be easily opened and closed . the plastic sheet that forms the container has a crease along the tear line , and has a narrow slit leading to the tear line . the lid tab extends axially outward beyond the join line by leaving a cutout when it is formed in the plastic sheet . however , it should be understood that the invention described herein should not be limited to the embodiments described herein . for example , in some embodiments the container systems can be used outside the context of a store clerk or a customer . for example , the packaging can be loaded and sealed at a manufacturing plant , prior to delivery to a store . alternatively , the packaging can be intended for a co - worker instead of a customer . further , although the embodiments described herein generally include a perforated join line that breaks upon opening the packaging , in some embodiments this feature can be absent . for example , in some embodiments it may be desirable to retain a hinging relationship between the lid and base of a container system , and thus the hinge would desirably not tear . however , the tearable join line can provide certain advantages , such as indicating that a container has previously been opened and the contents have potentially been contaminated . further variations on the described embodiments are also possible , such as alterations to the shape . it will be clear from the disclosure described herein that although most of the described embodiments are rectangular , other shapes are possible . as another example , the s shapes in the embodiment of fig1 - 16 can be substituted with another interlocking shape , such as a z - shape or a 90 - degree step . further , although the described embodiments are generally described in the context of deli containers , other contained consumer items are applicable . for example , in some embodiments the container systems can hold vegetables , nuts , candy , medical accessories , etc . although particular embodiments of the invention have been described and illustrated herein , it is recognized that modifications and variations may readily occur to those skilled in the art , and consequently , it is intended that the claims be interpreted to cover such modifications and equivalents .