Cement board or de-coupling or both - Tile Advice

[Duncan]

The question is in the title really
Do you guys tend to use one or the other or both
I know the jobs vary but just as a general rule?
Cheers
Duncan
Sorry if it's been asked before. I did search

 

[Flintstone]

Totally dependant on the situation and materials !

 

[Duncan]

Totally dependant on the situation and materials !

Well that might be true unless the answer is "No, I never use both" then it's not dependent on anything.

I'll rephrase it, what situations would make any of you guys use both?
And is it ever a bad idea