Twelve steady notes, three octaves. Tap one, it holds until you tap again — tune by ear or hand a choir its starting note.
A pitch pipe trains your ear; a tuner checks it. The classic workflow: sound the note, sing or tune toward it, then flip to the tuner and see how close your ear got. String players use the same trick with the drone player for intonation practice — a held reference exposes drift that a short beep hides.