Originally Posted by Ironfish653

Ah, I see you've been introduced to the compatibility curve. In terms of Shimano (I don't use Campy or SRAM) the compatibility follows the number of 'speeds' in the cassette. If you have a 9-sp cassette, you need a 9-sp shifter; doesn't matter if it's Sora, Tiagra, 105 or Ultegra, 9-sp is 9-sp. Now, you can also use a 10-sp derailleur (Not a 10S Mt RD or Tiagra 4700) with a 9-sp cassette and shifter; you'll still only have 9-speeds but it works fine. Likewise, you can put a 9-sp cassette on a 10-sp wheel, but not vice-versa (Freehubs are 8-9-10S compatible.You certainly can put a 10S cassette on a 9S wheel) ..