Derby fixtures

[tali1]

Which is the least played derby fixture?
And which was the first to reach 100 derby fixtures?
(English leagues only)

[mightyWBA]

Tranmere have only ever played Liverpool and Everton 4 times each
All the games were in the F A Cup or League Cup.
They've never met in a league fixture.

I suppose there are plenty of London clubs that haven't played each other much in competitive fixtures...Depends how a "derby" is defined :-)

I know the 100th local derby between WBA and Aston Vanilla took place in 1961