the example of slider of the invention overall has a structure known per se , close to that of fig1 . as with this known slider , the slider according to the invention has a cross - section in an inverse “ u ” shape , with a base 10 to which are attached two lateral and parallel wings ( or flanks ) 11 and 12 . in fig2 , it is presented in a position opposite to the one it occupies when it is in place on a sachet . the internal space of the slider encloses means adapted for stressing complementary profiled closure shapes or tracks of a sachet ( marked respectively p 1 , p 2 and s in fig1 ), by moving towards or moving away according to the direction of displacement of the slider along these profiled shapes or tracks p 1 and p 2 . the expression “ internal space ” means the space delimited by the base 10 and the flanks 11 and 12 . in this case , these means comprise a longitudinal central rib 13 c which extends according to the longitudinal median plane pm of the slider . this rib 13 c , originating from material with the inner face of the base 10 , separates the internal space into two channels or corridors c 1 and c 2 . as shown more particularly in fig3 and 5 , the rib 13 c extends downwards via an elongated central base 14 which overflows on either side of the rib 13 c to constitute guide facets of a protuberance t 1 , t 2 ( see fig1 ), here in the form of a point of an arrow , situated above an associated profiled closure shape p 1 , p 2 at the end of the sheets s 1 and s 2 of the bag . each facet is turned to the base 10 . “ protuberance ” denotes any means associated with the corresponding profiled shape , which allows the slider to cooperate with the sachet . this base 14 comprises a first “ wide ” part 140 , in the general form of an iron base ( that is , overall triangular when viewed from above ), which is prolonged by a narrower part 141 . also , opposite the two parts 140 and 141 of the base 14 , the inner faces of the flanks 11 and 12 each bear a projection 112 , 122 , respectively 113 , 123 , which is also provided with a guide facet of a protuberance . the corridors or channels c 1 and c 2 are therefore delimited by the base 10 , the rib 13 c , and its associated base 14 , the inner faces of the flanks 11 and 12 and their associated projections 112 , 122 , 113 , and 123 . in other terms , the corridors or channels c 1 and c 2 have the form of throats or grooves with convergent edges . in the example shown here , along the longitudinal free rim of each flank 11 and 12 extends a stiffening bar 110 , respectively 120 , of a thickness greater than the remaining part of the flank . in the example shown here , each bar 110 and 120 has on its free face , opposite the base 10 , a keying finger 111 , respectively 121 . in an embodiment which is not shown here , the slider could have the general structure described in french application published under no . 2 924 312 , in which load pins are provided in the extension of the flanks 11 and 12 . in the figures , it is evident that the opposite ends of the slider conform to arches 15 , of a thickness greater than the rest of its body . in accordance with the invention , the abovementioned corridors c 1 and c 2 define channels which are convergent towards an axial end ae 1 of the slider and are divergent towards the opposite axial end ae 2 . in other words said channels c 1 and c 2 extend generally according to respective median rectilinear planes y 1 - y ′ 1 and y 2 - y ′ 2 which are not parallel . a first axial end c 11 , c 21 of the channels c 1 and c 2 wherein the channels are convergent makes input end of the channels . this input end of the channels corresponds to the free opening defined between the central rib 13 c and the respective projections 113 , 123 . a second axial end c 12 , c 22 of the channels c 1 and c 2 wherein the channels are divergent makes output end of the channels . this output end of the channels corresponds to the free opening defined between the central rib 13 c and the respective projections 112 , 122 . moreover as indicated above and as illustrated on fig1 and 12 , the slider of the invention is characterized in that , the external edge ec 11 , ec 21 of the input end of the channels c 1 and c 2 is at a distance l 2 of the median central longitudinal plane pm at least equal to the distance l 1 separating the internal edge ic 12 , ic 22 of the output end of the channels c 1 and c 2 from the median central longitudinal plane pm and the wall of the base 10 in regard of the channels c 1 and c 2 corresponds to the lower thickness of the slider . as illustrated on fig1 , the external edge ec 11 , ec 21 of the input end of the channels c 1 and c 2 corresponds to the face of the projections 113 , 123 directed towards the median plane pm , while the internal edge ic 12 , ic 22 of the output end of the channels c 1 and c 2 corresponds to the face of the central rib 13 c directed towards the projections 112 , 122 . as explained above with the above feature , the present invention warrants that the two flanks 11 and 12 are pivoted along parallel axes x - x ′, which represent preferred fold lines . consequently the flanks 11 and 12 remain parallel when opening the slider . consequently inserting the slider upon the complementary tracks p 1 and p 2 is easier than with a slider in accordance with the prior art . preferentially the external edge ec 11 , ec 21 of the input end of the channels c 1 and c 2 is at a distance l 2 of the median central longitudinal plane pm slightly greater than the distance l 1 separating the internal edge ic 12 , ic 22 of the output end of the channels c 1 and c 2 from the median central longitudinal plane pm and the wall of the base 10 in regard of the channels c 1 and c 2 corresponds to the lower thickness of the slider . in practice , these corridors or channels c 1 and c 2 are bordered laterally by the base of the rib 13 c and by the foot of the projections 112 , 122 , 113 , and 123 . in an attempt to deform the slider according to the invention , in order to place profiled shapes or tracks such as those p 1 and p 2 of fig1 , the flanks 11 and 12 are moved apart from one another , for example by exerting a force on their inner face , as shown by arrows f of fig8 . this allows the corridors or channels c 1 and c 2 to be “ opened ” and enables placing of the profiled shapes or tracks p 1 and p 2 . once this operation is done , the force is stopped and the flanks 11 and 12 naturally regain their initial position . this force is applied most closely to the free end of the flanks 11 , 12 to produce the biggest possible lever arm . of course , the slider “ folds ” in the zones where there is least material . these “ fold zones ” correspond to the regions of the base 10 deprived of material outgrowth . these zones correspond to the corridors or channels c 1 and c 2 along axis x - x ′. the letter p in fig7 is used to mark the deformation planes of the base 10 of the slider , along axes x - x ′. with respect to parallel planes , the mechanical stresses to be implemented to produce these deformations are reasonable and the risks of breaking the slider are reduced . of course , the slider of the invention can be provided differently to that described here . it can especially be devoid of stiffening bars . in a variant embodiment , not shown , the above axes x - x ′ could be marked visually , for example in the form of a line traced on the inner face of the base 10 , or even in the form of a groove made therein .