Originally Posted by n4zou

Rear derailleurs come in two ratios, 1:1, which covers everything up to 8 cogs and 2:1 ratio for 9 and 10 speeds. With the introduction of the 9 speed cassettes 2:1 derailleurs had to be used due to the very close spacing of the cogs. The ratio is the travel the derailleur is pulled versus the rotation of the shifter. The space between the cogs is the same for 5 and 6 speeds and then changes for each of the 7, 8, and 9/10 speed freewheels/cassettes. A 5 speed index shifter will work fine with a 6 speed cog set as you may simply press the shifter lever up past the last index mark and get the sixth cog without any problems as long as you properly set the low limit stop on the derailleur as not to go to far and crash the chain into the spokes. Pretty much any index shifter for the number of cogs on the freewheel or cassette that matches the number of indexed shifter will function fine. Example: a 7-speed SRAM grip shift will work fine on a 7-speed Shimano freewheel or cassette. A 7-speed shifter will not work properly on a 6 or 8 speed freewheel or cassette.