32 in the two different positions and once the selection has been made fixed vertical cam 42a is bypassed under the action of spring 65 and flexible arm 7 of jack 4 and the two arm levers 5 tend to revert to their positions according to fig1 lower selectors 29a and 29b the upper selectors 32 of which are selected at 32a have their left ends advanced so that notches 67 abut contact faces 64a of lower selectors 29a which in turn come into contact with the broadened end 69 of the lower selectors 29b thus preventing two arm levers 5 from returning as represented in fig4 needle butts 66 of the corresponding pins will remain pushed inside needle bed 1 and thus be in position 8a of fig3 the corresponding needles 3 will be out of action lower selectors 29a for which the upper selectors 32 are selected at 32a will have their left ends pushed back and lower selectors 29b for which the upper selectors 32 are selected at 32b will have their left ends advanced so that notches 67 abut contact faces 64b as shown in fig6 their two arm levers 5 thus return to a medium position where they do not press any more on flexible arms 7 of jack 4 so that the corresponding needle butts 8 completely come out of needle bed 1 and will be engaged by the cams.