to portions 3 of the tubular knitted fabric 1 intended to form the thigh passage an oblique cut 18 allows to form from the piece 17 two pieces 15 and 15 u2032 symmetrical in rectangular trapezia the piece 15 represented on fig7 includes in addition to its reinforced edge 23 parallel edges 1510 and 1513 and an oblique edge 1511 we then go to the sewing of the crotch pieces 15 15 u2032 on the knitted tube for that each incision 7 7 u2032 is laterally spaced apart in the transverse direction so as to put the edges 10 11 and 10 u2032 11 u2032 substantially in the shape represented in fig8 where they are at an obtuse angle together always with the tab 14 therebetween and the edge 16 thereof at low portion these elements form a broken front bottom line 10 11 16 11 u2032 10 u2032 in which the edge 16 is a low edge and the portions 10 11 11 u2032 10 u2032 a top edge with respect to the low edge in the same time the incision 8 is spaced apart so as to put the edges 12 13 substantially in the extension of one another as represented so as to form a practically rectilinear rear bottom line 12 13 the low edge 16 is then sewn in the middle of the rear bottom line 12 13 in the portion facing the low edge 16 and the lateral crotch trapezia 15 15 u2032 are attached and sewn between the lateral portions of.