Relay question about diodes across the coil

Back when I was a kid learning about relay circuits on the job
(1960's), most of the control circuits I got involved in used low
voltage A.C. and I don't ever remember seeing a diode. I just found
out that it's good practice to put a diode across the coil of relays
to keep them from chattering due to the current stored in the coil
inductance. Question is: Only on D.C. relays or is this supposed
to be done on A.C. relays as well?

Jeff