What's it for?

[ossettjohn]

My volume pedal has a 3rd socket labelled Tuner. What is it for?

Sorry if it's a stupid question but if you don't ask...

[john roche]

You can connect your tuner to the VP while playing, you can then check your tuning as you play...

[Uncle Roy]

There is no stupid steel questions on this site. Good on you. That's part of the reason for sites like this to get all the advice you need. Keep the questions coming there's a bunch of clever guys on here who are always willing to help.

Tuners " usually" modify the sound of the instrument
Specific Pro Volume pedals have a 2nd output - which is most often used for the tuner
the poor bugger's out on his own rather than having him " in the chain "