fig1 shows how a backrest a according to the present invention can be placed against the conventional backrest of a chair b . the backrest a can easily be built as an integral component of chair b . in the embodiment shown in fig2 the backrest comprises a rigid base plate 10 , e . g . made of metal , wood or hard plastic . on this base plate 10 there is a support member 20 positioned by means of two straps 11 , 12 , which form the support member &# 39 ; s two connection positions . the support member consists of a sheet of pliable plastic , which is loosely inserted under straps 11 , 12 . between base plate 10 and support member 20 , in the space between straps 11 and 12 , there is a distance member in the form of a strip 13 having a cross - section in the shape of an isosceles triangle , movably arranged . by moving distance member 13 towards strap 11 or towards strap 12 and / or by turning over distance member 13 , the profile of the support member 20 is altered . the small changes which then occur in the length of the part of support member 20 between the two straps are compensated by this member not being fixed to at least one of straps 11 , 12 . assuming that support member 20 is nailed to strap 11 , continuing compensation movements shown by the double arrow p of strap 12 are possible . however , support member 20 can also , in principle , freely pass under both straps , as shown in the drawing . in the embodiment according to fig2 support member 20 is joined at both connecting positions to base plate 10 in an adjustable and non - pivotable manner . the shape of the profile of support member 20 , if distance member 13 is central between straps 11 , 12 , is symmetrical in relation to this distance member . however , one of the ends of the support member maybe fixed , e . g . according to fig3 by means of a spindle or pivot 17 . then an asymmetrical form is obtained . the whole backrest can be placed on a chair b either with strap 12 or the spindle 17 uppermost . it is obvious that a certain asymmetry is also obtained if straps 11 , 12 of fig2 are of different widths . in the embodiment according to fig4 the support member consists of a number of continuous longitudinal bands 20 &# 39 ;, of the same material as plate 20 . straps 11 and 12 have been changed to clamp bridges 11 &# 39 ; and 12 &# 39 ; for the separate bands 20 &# 39 ;. as is shown the clamp bridges are integral pressed out strips of the base plate . bands 20 &# 39 ; can , according to fig5 be held in a simple manner at one of the connecting positions by their end parts being curved and provided with notches 21 , which in principle are of the same width as clamp bridges 11 &# 39 ;. said end parts can consequently be hooked on to the clamp bridges and again easily be removed , e . g . for cleaning purposes . according to fig6 the distance member 13 consists of a narrow , longitudinally continuous strip of , e . g ., sheet metal or the like , which is bent to a v - shape with different dimensions c and d , and at both ends is provided with bearing pins 15 for rotatably engaging in brackets . these brackets preferably consist of the turned - up ends of a flat , displaceable , continuous strip 24 , e . g . made of a strip metal , under the base plate 10 , formed with keyhole - shaped openings 27 . the wider parts of the openings , but not the narrower , allow passage of the heads 15a of bearing pins 15 . the strip or band 24 is furthermore provided at its ends , in the upwardly - bent parts , with resilient tongues 25 , which in turn are provided with abutment members 26 in the form of projections , pins or the like for positioning along the base plate 10 . this base plate is provided along the edge m ( fig4 ), for the positioning of the distance member and as shown in fig7 with a number of notches 16 for the abutment members 26 . if the base plate is made of sheet metal , these notches can be arranged to advantage as an upwardly - and downwardly - folded edge area 10a , which at the same time holds the distance member 13 in place laterally . fig7 shows a piece cut out of the side of the base plate 10 showing that over a length m ( fig4 ) the edge of the base plate is bent . the right hand edge of fig7 is the right hand longitudinal edge of the base plate . a strip 10a is pressed out of the base plate near the edge to form an upwardly and downwardly folded part . the downwardly folded part is provided with notches 16 . the heads 15a of the distance member 13 ( fig6 ) are entered into the lower larger ends of the openings 27 and moved up to bring the pins 15 into the narrow upper ends of the openings 27 . thus the member 13 is attached to the bracket 24 . the bracket then is presented to the base plate 10 and the members 26 enter a pair of opposed notches 16 . the member 13 has projections 18 to be held by the fingers when positioning the member 13 . according to fig8 a rigid base plate 10 &# 39 ; is curved transversely and has a radius of curvature r . the support member consists of a plurality of resilient bands 20 &# 39 ; spaced apart a distance d as in fig4 fixed at set of adjacent ends by bolts 21 to the base plate and passing at their other ends freely under a strap 12 . the distance member is defined by a doubly curved rod 13 &# 39 ; having a rectangular cross - section . the rod 13 &# 39 ; is curved with the same radius r as the base plate 10 &# 39 ; as well in the direction of the arrow p 1 , perpendicular to the long side ( which has a length b ) of the cross - section , as in the direction of the arrow p 2 , perpendicular to the short side ( which has a length a ) of the cross - section . the edges of the rod 13 &# 39 ;, such as edge 13 &# 39 ; a , are therefore defined by the intersection line of two cylinders having both the radius r and mutually perpendicular axes . from the side faces of the rod 13 &# 39 ; protrude short shafts 15 &# 39 ; on which manipulating knobs 15a &# 39 ; are mounted ( only one knob 15a &# 39 ; is shown for clarity ). along the rectilinear longitudinal sides of the base plate 10 &# 39 ; protruding edges 10a &# 39 ; with a plurality of recesses 16 &# 39 ; for the shafts 15 &# 39 ; are arranged . by turning the knobs 15a &# 39 ;, one or the other of the distances a , b is chosen as the height dimension of the distance member 13 &# 39 ;. the rod 13 &# 39 ; will however in both chosen positions along its entire underface rest on the base plate 10 &# 39 ;, and the bands 20 &# 39 ; in their turn will be sustained by a concavely curved face of the distance member , so that a saddle - shaped back rest surface in the manner of a car seat is obtained . the back of the curved base plate 10 &# 39 ; fits in its turn better in a conventional , also transversely curved , back rest . the resilience of the bands 20 &# 39 ; presses the distance member 13 &# 39 ; with its shafts 15 &# 39 ; into a chosen pair of recesses 16 &# 39 ;. this resilience can however easily be overcome for re - setting the distance member 13 &# 39 ; in another pair of recesses 16 &# 39 ;. there is in practice no need for arranging a particular stop to prevent the distance member from being turned &# 34 ; upside down &# 34 ;, i . e . with a concave face towards the base plate 10 &# 39 ;, because such a position is so obviously incorrect and is not useful , that it immediately would be abandoned if set by mistake . according to fig9 the shafts 15 &# 39 ; are not attached in the geometrical center of the rectangular end faces of the rod 13 &# 39 ;, but at equal distances from three sides to the height of the centres of the recesses 16 &# 39 ; above the upper face of the base plate 10 &# 39 ;. it will be however realised that in principle the shafts 15 &# 39 ; can be attached even in the geometrical center of a rectangular end face of the rod 13 &# 39 ;, the recesses 16 &# 39 ; in accordance with fig1 being provided with elongated edges 16 &# 34 ; extending at right angles to the base plate 10 &# 39 ;. the shaft 15 will then either rest close to the bottom 16 &# 34 ; a of the recess which has the shape of an elongated slot , or at some distance above this bottom position . it will be further understood , in this context , that a double - curved distance member doesn &# 39 ; t necessarily need to be associated with a transversely curved base plate as 10 &# 39 ;, because according to fig1 sufficiently high protruding edges 10a &# 39 ; can support such a distance member in any desired distance above a plane base plate such as 10 . the shafts 15 &# 34 ; and the recesses 16 &# 34 ; may then , and also in any other case , according to fig1 have other than circular cross - section to better secure the chosen position of the distance member . this position is changed by the distance member 13 &# 39 ; being lifted out of the respective pair of recesses or slits 16 &# 34 ;, in the same way as when moving it into another pair of recesses or slits , turned with 90 °, and reset . in fig1 a base plate 10 is shown to which a plurality of resilient bands 20 &# 39 ; are fixedly attached at both ends by means of bolts 21 and 22 . the length of the bands 20 between the bolts 21 and 22 is longer than the distance between these bolts on the base plate 10 , so that a convex curvature of the bands 20 &# 39 ; is obtained without any particular distance element . to make this curvature adjustable , a strap 12 &# 39 ; overbridging the bands 20 &# 39 ; is movable with the aid of bent end portions provided with inwardly protruding engagement means 12b , in longitudinal grooves 19 in the side edges of the base plate 10 where a plurality of recesses 19a are provided to receive the inwardly protruding engagement means 12b . alternatively , according to fig1 , the bands 20 &# 39 ; can be fixed by bolts 12a to the strap 12 &# 39 ; or according to fig1 , the bands 20 &# 39 ; may have a plurality of openings 20a , longitudinally spaced apart , through which bolts 22 &# 39 ; ( fig1 ) can be passed and anchored in corresponding openings in the base plate 10 and / or in the fixed strap 12 . according to fig1 , the bands 20 &# 39 ; can be attached to the base plate 10 &# 39 ; rotatably , as shown at 17 , and several positions such as 17 &# 39 ;, 17 &# 34 ; may be chosen to obtain various curvatures . at the other end , either also a rotatable attachment , or an attachment with face - to - face contact , as shown in fig1 , may be chosen . according to fig1 , a support member 20a may in the embodiments according to fig1 to 14 have in the longitudinal cross - section a zone 20b with greater resiliency than in the neighbouring zones 20c . the top of the curvature will then always lie in the zone 20b . preferably , among the several possible setting positions for a support member , one is provided where the support member along its whole length adheres to the base plate 10 , i . e . where the distances between the attachment positions on the support member and on the base plate are equal . this is the transport and storing position of the back rest , shown in fig1 . the base plate in the embodiments according to fig1 to 14 can be made flexible in the transverse direction only , to adapt itself to the transverse curvature e . g . of an underlying conventional back rest . this flexibility in the transverse direction only may be achieved e . g . by making the base plate of an at least partially resilient material and either giving it in transverse cross - section a profile analogous to that shown in fig1 , or providing it in accordance with fig1 with longitudinal stiffening ribs 10a counter - acting the inherent resiliency in the longitudinal direction . the strap 12 is then also made of resilient material . in fig1 a base plate 10 &# 39 ; for the back rest according to the present invention is shown in plan view , and in fig1 , in the left hand part thereof , in a section along the line m -- m , and in the right hand part thereof in a section along the line n -- n in fig1 . the base plate 10 &# 39 ; is made of non - resilient plastic and reinforced by ribs such as 110 which radiate from a central hub 111 and at their free ends merge into the concave upper face of the base plate . near both transversal ends of the base plate a series of loops or straps 112 is arranged under which bands 20 &# 39 ; ( fig8 ) can be slid . along the whole periphery of the base plate which at the longitudinally central part is somewhat drawn - in , an upturned edge 10 &# 39 ; is arranged . the base plate according to fig1 can be used as well for back rests with a distance member , as for back rests without a distance member . for the former case , a series of recesses or notches 16 &# 39 ; is arranged in the longitudinally central part of the upturned edge 10a &# 39 ; for the short shafts 15 &# 39 ; ( fig1 ). the manipulating knobs 15a &# 39 ; do not protrude out of the periphery of the base plate thanks to the indrawn central area . bands 20 &# 39 ; longer than the mutual distance between the upturned edges at the two shorter sides are slid under the loops or strips 112 . their ends abut then against said upturned edges at the two shorter sides and a curvature of the bands is obtained . many variants are possible within the general scope of the invention . for example , the distance member can be of other than the two shown sections and the connection of the support member to the base plate can be formed in a different manner .
a chair backrest having a rigid rectangular base plate and a flexible back support member attached to but movable relative to the base plate , and a spacer between the base plate and the support member , which spacer is movable to adjust the contour of the support member , cooperating locking members being provided on the base plate and spacer to lock the spacer in a number of chosen positions .