26 shifter 20 is guided by shaft 22 and shifter 28 is guided by shaft 30 the master shifters 20 28 may be of the type disclosed in fig4 of u s pat no 4 162 643 referring to fig2 there is shown mating heads 34 38 on the shafts 16 18 respectively head 34 supports a blade 32 which mates with blade 36 on head 38 shaft 16 also supports a head 42 which mates with head 46 head 42 has a blade 40 which mates with a blade 44 on head 46 the thusly described sets of mating heads and blades may be referred to as the operative heads of the shafts 16 and 18 thus shafts 16 and 18 are sufficiently long so as to have an operative zone corresponding in length to the widest web capable of being slit by the apparatus 10 as well as an idle or park zone indicated at the right hand side of fig2 in the idle or park zone there are mating sets of heads and blades designated 48 50 and 52 a greater or lesser number of idle sets of mating heads and blades may exist during any particular production run each of the heads has a radially disposed side face which constitutes a pushing surface so that mating sets of heads may be pushed along their respective shafts in the manner described in u s pat no 4 162 643 thus in the preferred embodiment each head has a pushing surface on opposite ends thereof so that the heads may be pushed in opposite directions preferably all of the heads are of the same size except for the outermost head on one of the shafts which is wider than.