A chromatic tuner does not assume an instrument or a tuning: it names whatever note it hears, out of all twelve, and shows how far it is from perfect in cents. That makes it the right tool for unusual tunings, for singers, for wind instruments, and for checking single fretted notes on a guitar.
Set the reference pitch anywhere from 415 to 466 Hz. Most music uses A = 440 Hz, but some orchestras tune to 442 or 443, baroque ensembles often use 415, and older recordings can sit between notes. Type the exact value or nudge it with the arrow keys.
Players of transposing instruments can switch the display to written notes. Choose B♭ for trumpet and clarinet, E♭ for alto saxophone, F for horn and more, and the tuner shows the note on your page while still telling you the sounding pitch underneath.
Pick sharps, flats or both for note names, choose the needle, the strobe or both, and use the reference keyboard to hear any pitch on a real guitar sound. Hold keeps the last reading on screen after the note fades, so you can look up from the instrument.