[ 0023 ] fig1 shows in perspective and somewhat schematically a stretcher bar frame 10 for receiving a canvas . fig1 is only for purposes of illustration ; the frames with which the invention is concerned are generally of somewhat different proportion , with at least one dimension which is quite long so as to require bracing to prevent the stretcher bars 12 and 14 from pulling inwardly under the tension of a stretched canvas . [ 0024 ] fig1 shows a groove 16 which is provided in each stretcher bar , on the inner surface such that it faces inwardly in the completed stretcher bar frame 10 , forming a contiguous groove around the inner periphery of the frame . preferably this groove 16 is located off center on the inner surface 18 of each stretcher bar , either toward the canvas at the front of the frame or toward the back of the frame . this is to accompany a preferred embodiment of the invention which will be described below . [ 0025 ] fig1 also shows recesses 20 and 22 which are portions of mortises , of the mortis - and - tenon joinery preferably used at the mitered edges of the stretcher bars of the frame . [ 0026 ] fig2 a through 2t show various configuration of bracing according to the system of the invention , with the bracing in different patterns on frames , for purpose of example , that have essentially the same proportions . fig2 a , for example , shows a stretcher bar frame 10 having side and end stretcher bars 12 and 14 , and having a single , simple brace 24 spanning across the shorter dimension , so as to maintain separation between the longer stretcher bars 14 under conditions of canvas tension . [ 0027 ] fig2 b shows a similar frame 10 b with three simple braces 24 supporting the longer stretcher bars against pulling inwardly . [ 0028 ] fig2 c and 2d are similar to fig2 a and 2b but show that the simple braces 24 a can be of greater width when needed . [ 0029 ] fig2 e and 2f show the use of a simple brace 24 a crossing the narrow dimension of a frame , while braces 26 in the perpendicular direction are provided for bracing the end stretcher bars 14 . the braces 26 , which can be called cross braces , do not extend the full length between the end stretcher bar 14 but extend from a stretcher bar to the simple brace 24 a . [ 0030 ] fig2 g and 2i are included to show that the simple brace 24 g or 24 i can either be positioned in the horizontal direction or the vertical direction , with the cross braces 26 g or 26 i being perpendicular . in each of these drawings , the braces are shown with continuous lines for the brace is continuous , and cross braces are shown abutting against the continuous braces . [ 0031 ] fig2 p through 2t show diamond braces or angled braces 30 near corners of the stretcher bar frames . these can be used alone , as in fig2 p , or in combination with other bracing as in the other drawings . fig2 q shows four diamond braces 30 and a simple brace 24 extending across the narrower dimension of the frame , as does fig2 r . fig2 s and 2t show other patterns . [ 0032 ] fig3 shows another exemplary frame pattern wherein a stretcher bar frame 32 has a set of braces none of which is a “ simple ” brace extending the entire length between parallel stretcher bars . this differs from the arrangements shown in the figures above . [ 0033 ] fig4 shows one preferred arrangement for attachment of the braces to the stretcher bars . a simple brace 24 is shown extending between two parallel stretcher bars 14 , the stretcher bars being shown in cross section . the stretcher bar grooves are shown at 16 . the grooves receive a tenon 34 which extends from each end of the simple brace 24 . fig4 illustrates the reason why the stretcher bar groove 16 is located either toward the front of the frame or back of the frame , rather than centrally located along the width of the stretcher bar . the tenons 34 preferably are at one edge of the brace 24 as shown . the brace has an adjacent end 36 which does not protrude as does the tenon , and which abuts against the face of the stretcher bar 14 as shown . between the end 36 and the tenon 34 is preferably included a groove 38 , which is known from previous canvas frame braces . the groove 38 allows a wedge - shaped key 40 to be tapped in with a hammer when the frame needs to be expanded for re - tensioning the canvas . [ 0034 ] fig8 shows the simple brace of fig4 . the narrow dimension of the brace is shown , and as noted above , the wide dimension can be in several different widths if desired , as illustrated in fig2 a - 2 t . [ 0035 ] fig9 shows a different brace 26 , which can be called a cross brace as noted above . this brace is similar to the simple brace 24 , but it is not intended the full length from stretcher bar to stretcher bar , and thus has a different second end 42 . the second end 42 has a tenon 44 at its center , designed to engage with a groove in another brace . the groove is shown , for example , at 46 in the cross brace 26 of fig9 and it should be assumed that all braces shown herein can have such grooves , although certain braces dedicated to not receiving any further bracing can be formed without the grooves . this groove 46 is also shown in fig8 . [ 0036 ] fig1 shows a further type of brace 48 , which can be called a middle brace . such a brace would be at a position such as shown at 48 or 48 a in fig2 n and 2o , for example , or at 48 a in fig2 t . neither end of the middle brace 48 engages against a stretcher bar , but the brace merely extends between two parallel braces . again , these braces may or may not have grooves for receiving further braces . [ 0037 ] fig6 shows a corner support 50 which can be used with frames according to the invention . the corner support 50 , which can be formed with hardboard , extends into the groove 16 of both the stretcher bars 14 and 16 at the corner formed between them , and screws 52 can be used to lock this corner support in position . this illustrates another useful purpose of the groove 16 , which extends around the inner periphery of the stretcher bar frame . fig6 also shows keys or wedges 40 which can be used to lock the frame size once the frame has been expanded ( as shown at 54 , for example ) to re - tension a canvas . these wedges are jammed not into the groove 16 but into mortises which a part of mortis - and - tenon joinery at the mitered corners of the frame , as further explained below . [ 0038 ] fig1 shows another type of angled brace 56 or diamond brace 30 , also shown in fig2 p - 2 t . this illustrates the use of the groove 16 to receive this type of brace . again , a screw 52 can be used on the stretcher bars at each end of the diamond brace 30 , to lock the brace in position . fig7 shows a corner support 55 as used between a brace 24 or 26 and a stretcher bar 12 or 14 . the corner support is tenoned as shown at 58 , 60 and 62 so that it enters the groove 46 of the brace and the groove 16 of the stretcher bar . gluing can be used between internal members of the brace system , but preferably braces are not glued to stretcher bars , since the stretcher bar frame may need to be expanded for re - tensioning of a canvas . [ 0040 ] fig1 shows a portion of a stretcher bar 14 , having a double mortise - and - tenon joinery configuration . typically stretcher bars have a single mortise 66 and a single tenon 68 , positioned to be engaged with a complementarily placed tenon - and - mortise in another stretcher bar which is mitered together with the bar 14 at a right angle . in this aspect of the present invention , the end of a stretcher bar 14 has a double mortise and tenon , with an additional tenon 68 and an additional mortise 66 as shown . these cooperate with complementarily placed mortises and tenons on the stretcher bar end to be joined together with the bar 14 . these joints typically are not glued but are merely held together by friction , and the doubling of the mortise - and - tenon connection adds further stability and strength to the connection . although , as stated above , the braces of the invention , i . e . the simple braces or the cross braces which have at least one end engaging with a stretcher bar , have their tenons 34 at one edge of the brace , these could be centered in the end of the braces if desired , in the same manner as the tenon 44 seen in fig9 or the tenons 49 in fig1 . as noted above in the discussion of fig4 the principal reason for the offset of the tenons in the main braces 24 is to accommodate the brace groove 38 which is useful when the frame is expanded for re - tensioning a canvas . however , modularity of the invention can be served by having all braces with the tenons centered . in that case , a different type of expansion wedge can be used between the simple braces and the adjoining stretcher bars , i . e . a forked wedge ( not shown ) having two wedge sections joined together in a generally u - shape . in this way , the main braces or simple braces described above can be interchangeable with cross braces or middle braces for better modularity in assembling braced frames of various sizes and configurations . in the case of centered tenons on all braces , the groove 16 in the stretcher bars would be moved more toward the center of the width of the bar . variations to the preferred embodiments are possible . for example , the braces and / or the stretcher bars could have ridges rather than grooves , with the ends of braces having a groove or mortise to engage with the ridge . the grooves or ridges on the bars or on the braces can be referred to as engagement tracks , and the corresponding engagement structure ( tab or groove ) on the brace ends can be referred to as cooperative engagement structure . the above described preferred embodiments are intended to illustrate the principles of the invention , but not to limit its scope . other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims .