referring now to fig1 the utility knife of the present invention is depicted in a preferred embodiment wherein the knife includes a retractable blade having a trapezoidal shape . however , it will be apparent to those skilled in this art that the invention also has applicability to non - retractable , fixed - blade knives as well . further , the blade may take a number of forms including the shape illustrated , a hook blade , a scoring blade , or any other form of blade which can be adapted for mounting to the blade carrier . as shown , knife 10 comprises a pair of complementary , mating and opposing knife halves 12 and 14 which are assembled using screw 16 to form an exterior profile which is easy to grip in the hand of a user . the mating knife halves 12 and 14 form a hollow interior which provides space for the blade carrier 18 and blade 20 ( shown in phantom lines ) to reside . as explained in greater detail below , knife 10 may include the optional feature of a retractable blade . where blade retractability is desired , within the interior of knife halves 12 and 14 are spaced , substantially parallel guides 22 and 24 between which blade carrier 18 is mounted for linear longitudinal movement . blade 20 is replaceable as is conventional in this art by removing screw 16 and taking apart the knife halves . knife halves 12 and 14 mate to form a forward blade opening 26 to provide an opening for the blade 20 to be moved longitudinally from a retracted , sheathed position to an extended cutting position ( shown ). as is also conventional , there may be provided additional intermediate positions for the blade carrier and blade . in describing the knife and its components , the convention which will be used will be to refer to the blade end of the knife as the forward end ( left - facing end in fig1 ). blade carrier 18 includes a generally flat blade support portion 28 on which blade 20 rests and a blade retaining flange 30 for retaining one edge of the blade . as shown , blade retaining flange 30 extends at a right angle from blade support portion 28 and rests on guide 22 . the cutting edge of blade 20 rests on flange 30 . a protruding edge 32 also projects from blade support portion 28 to engage the rear edge of blade 20 . as shown , edge 32 is angled to mate with the angle of the rear edge of blade 20 . a second flange 34 , extending from blade support portion 28 , engages the top edge of blade 20 so that blade 20 fits between the two flanges . flange 34 rests against guide 24 . blade 20 includes at least one , and preferably two , locating notches 36 and 38 ( best seen in fig3 ) which are sized to fit against posts 40 and 42 , respectively . typically , the notches are semi - circular in shape and mate with the semi - cylindrical shape of the posts . posts 40 and 42 extend upwardly from blade support portion 28 and terminate at the surface of flange 34 . their rear surfaces are integral with and form a part of flange 34 . when blade 20 and notches 36 and 38 are fitted into position to engage posts 40 and 42 , blade 20 is restrained from longitudinal movement ( i . e ., movement to the left or right as shown in fig1 ) in blade carrier 18 . to insert the blade 20 in the blade carrier 18 , the upper edge of the blade 20 which has the notches 36 and 38 is held so that it is angled downwardly against blade carrier 18 to mate with the lower portions 44 and 46 of posts 40 and 42 . the blade is then allowed to drop into position against carrier 18 . to remove the blade , the end of the blade is grasped , and the cutting edge is tilted upwardly until it clears retaining flange 30 . the blade is then pulled away from posts 40 and 42 . in the present invention , posts 40 and 42 also perform a second function , that of restraining blade 20 from lateral movement ( i . e ., movement away from the surface of blade support portion 28 ) when blade 20 is subjected to lateral ( i . e ., side - to - side ) and / or twisting ( i . e ., where the top portion of the blade and the cutting edge are subjected to forces in opposite directions ) forces . in some prior art knife designs , lateral or twisting forces encountered by the blade during heavy use would cause the blade to move away from the surface of the blade support . the notches in the blade would move upwardly along the posts , and the blade would be released from the lateral restraints of those posts . the present invention solves that problem by providing one or more posts which have a profile which engages the notch or notches 36 and 38 on blade 20 when the blade is subjected to a lateral or twisting force . in the embodiment shown in fig2 and 3 , where like reference numerals represent like elements , the lower portions 44 and 46 ( i . e ., those portions nearest the surface of blade support portion 28 ) of posts 40 and 42 have a generally semi - circular cross - section which mates with semi - circular notches 36 and 38 on blade 20 . those lower portions have a vertical length which is somewhat greater than the thickness of blade 20 to permit easy insertion and withdrawal of the blade during replacement . however , the upper portions 48 and 50 of posts 40 and 42 have , as shown , an increasingly thickening profile along their respective facing surfaces . this thicker profile results in a cross - section which has a diameter larger than the diameter of notches 36 and 38 . while the thickening profile is depicted as a relatively smooth transition between lower and upper portions , it will be apparent that the changing profile need not be continuous or even smooth as long as the profile at some point has a diameter larger than that of the notches on the blade . when blade 20 is subjected to lateral or twisting forces , any lateral movement of the blade is restrained by the frictional engagement of notches 36 and 38 with upper post portions 48 and 50 . thus , the posts perform a second function , that of preventing blade 20 from disengaging from the blade carrier 18 during use of the knife without the need for separate overhanging tabs or flaps on the blade carrier or protrusions on the facing blade half . in a preferred embodiment of the invention , blade carrier 18 , including blade support portion 28 , flanges 30 and 34 , as well as posts 40 and 42 are all cast as a single , integral construction and may be fabricated from any suitable casting metal such as zinc . as shown in fig2 and 3 , an open area 52 defined by parting line 54 is placed in the casting to permit the increased profile thicknesses in posts 40 and 42 to be cast . referring now to fig4 an alternative embodiment of the invention is shown . in this embodiment , the upper portions 48 and 50 of posts 40 and 42 have a thickened profile along opposing faces of the posts . to permit such profiles to be cast , open areas 52 are provided in the surface of blade carrier 18 . fig5 shows yet another embodiment of the invention . in this embodiment , the upper portions 48 and 50 of posts 40 and 42 have a thickened profile along the rearward ( i . e ., facing away from blade opening 26 ) faces of the posts . again , to permit such profiles to be cast , open areas 52 are provided in the surface of blade carrier 18 . referring now to fig6 yet another embodiment of the invention is illustrated . in this embodiment , at least post 42 ( the more rearward of the two posts ) has an increased length which is sufficient to maintain engagement with notch 38 on blade 20 even when blade 20 is subjected to severe lateral or twisting forces . thus , as shown , posts 40 and 42 have lengths which extend beyond the surface of flange 34 and toward the inwardly facing body of mating knife half 12 . preferably , the posts extend so that they will just clear the inwardly facing body of knife half 12 . where a retractable blade is provided , post 40 has a somewhat shorter length than post 42 and may have a beveled leading edge as shown so that when the blade carrier is moved to an extended cutting position for the blade , posts 40 and 42 will clear the interior of the opposite half of the knife as it tapers toward forward blade opening 26 . while it will be apparent that the exact lengths of the posts will differ depending upon the size and shape of the knife and blade carrier , in one example post 42 has a length of 0 . 245 inches , and post 40 has a length of 0 . 209 inches with a bevel angle of 15 ° for the leading edge . referring back now to fig1 an optional feature of the present invention is to provide a retractable blade for the knife . as shown , guide 24 includes beneath it a series of lands 56 and 58 having notches , such as notch 60 therebetween . a thumb button 62 is connected to a latch 64 which extends into notch 60 . a coil spring 66 , restrained in blade carrier 18 , normally biases latch 64 into notch 60 to prevent longitudinal movement of blade carrier 18 and blade 20 . however , when thumb button 62 is depressed by the knife operator , latch 64 is also depressed so that it clears notch 60 . by pushing thumb button 62 longitudinally when depressed , blade 20 is released for longitudinal movement as blade carrier 18 moves along guides 22 and 24 , such as from a retracted position to an extended cutting position . while certain representative embodiments and details have been shown for purposes of illustrating the invention , it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention , which is defined in the appended claims .