rear shifters have 1 less click than the number of speeds. The positions are start + X number of shifts totaling x+1 speeds.
front shifters are trickier, they can have many clicks, like a micro-shift system, allowing user trim, 1 for a double or two for a triple. However some have 2 clicks close together to allow trim adjustments on the outer chainring.
If your rear shifter is able to access all the rear sprockets, and still have one more position, then I'd suspect that you have one made for 9s with an 8s system, but there may be another reason.
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Last edited by FBinNY; 09-13-13 at 11:27 AM.