Tapping a Bipolar Power Supply

Greetings fellows, I have a center tapped transformer providing _+15 bipolar power for some audio circuits using op amps. So I have

+15volts
Ground
_15volts

I also need to add an adjustable voltage regulator to provide power for other 12v components in the same circuit.

Where do I tap the power rails, between +15 and _15, or do I tap between +15 and ground?

Thanks!

+15 and ground