An after-market bridge that prevents your strings from detuning.
https://www.guitarworld.com/gear/accessories/evertune-pros-and-cons
'“Ron Nevison was doing a session in LA and needed an EverTune Telecaster,” Blackett says.
“I knew Cosmos was a huge fan, so I said, ‘Do you mind loaning him your guitar for a session?’ He goes, ‘Sure, but I haven't played it in a while. It's going to need a setup.’ While we’re on the phone, he pulls it out of the case, hits a chord – and it’s perfectly in tune.'
@elsemusic It’s an amazingly clever system. I had a long chat with a luthier about it some time ago, and we both agreed that for my playing style it would get in the way more than anything. I (like many) still wanted tuning stability, and went for a carbon fibre guitar instead. I’ve probably sent a handful of people to EverTune so far though.