The classic- 1/4" Phone jack- vintage Switchcraft style, chromed steel contacts- But stereo- perfect for stereo wiring jobs- But we use them as an automatic switch for guitars with 9 volt battery circuits- It automatically shorts the ground wire of the 9 volt battery to guitar ground- presto! Instant switch. These are very tough jacks that really take a licking- and seem to survive all manner of soldering iron mishaps!
I've sold a ton of these- and they're flat-out great. Totally reliable and really nicely made!