Derailleur/shifter mix and match?

[Okeydokey]

If I have a seven gear cassette, can I use a nine gear/derailleur/shifter? I can see why not the other way round. I see on ebay mention of 1:1 derailleurs and am not sure if that means every time the shifter clicks there is a set dimension to the movement of the derailleur. Is there a generic alternative perhaps?

 

[TheDoctor]

With a 7 speed cassette, you can use a 8 speed mech for sure, probably a 9 speed and probably not a ten speed. 8 speed shifter should work, 9 or more definitely not. I'm assuming this is Shimano - SRAM shifters are different and only work with SRAM mechs. Except for the Shimano compatible SRAM gears and shifters, but I'm ignoring those.

 

[Okeydokey]

Probably 9 will do! Thanks Doc.