Here's what D'Addario say about the Micro Headstock Tuner Twin pack:
The D'Addario Micro Headstock Tuner is a feature-packed tuner your audience will never see. This highly accurate tuner features a full-color display with four selectable viewing angles (two vertical, two horizontal) in a housing that blends with the aesthetics of any instrument.
The advanced tuning algorithms allow you to tune in noisy environments, while the ratchet clamp securely attaches to the instrument to hang tight during live performance. It’s so small, convenient, and accurate, you’ll want one for every instrument you own.
"The Micro Headstock Tuner is invaluable. It's barely noticeable on the guitar and clear as day to see on stage. Super handy!" - Rabea Massaad