A trumpet player sits down at a piano, plays the note their sheet music calls C, and a B♭ comes out of the piano's speaker. Not a mistuned C. A different note, a whole step down, every single time. If you've never met this before it looks like either the trumpet or the music is broken. Neither is. This is called transposition, it's on purpose, and it's the reason our tuner shows two note names at once for four of its instrument presets.
Here's the part that actually matters: the trumpet isn't playing a wrong pitch, and the pitch itself never moves. Press the same three valves, blow the same partial, and the sound wave coming out of the bell is a fixed frequency no matter what anyone calls it. What changes is which name we've agreed to put on the page. A B♭ trumpet's page calls that frequency "C." A piano's page, or a flute's page, calls the identical frequency "B♭." Both are correct, because "correct" here just means "matches the convention that instrument was built around."
That convention has a name: transposition. An instrument is "transposing" when the note printed in its part is not the pitch a fixed-pitch instrument like a piano would call by that name. B♭ trumpet, B♭ clarinet, E♭ alto sax, F horn — all transposing. Flute, oboe, trombone, tuba, violin, voice — none of them are. They're "concert pitch," meaning written note and sounding note are the same thing.
Before valves existed, a trumpet or horn could only play the notes of one overtone series, full stop — no in-between pitches, no chromatic runs. Want to play in a different key? Swap in a different length of tubing (a "crook"), which shifts the whole overtone series up or down. Composers wrote a single fingering chart and let the player fit whichever crook the key demanded, so the same written C always meant "the tonic note of whatever key we're in today," regardless of the actual frequency.
Valves killed the need to swap tubing mid-concert, but the notation habit was already 150 years old and every existing part was printed the old way. Nobody reprinted the entire orchestral and band repertoire. The convention just calcified: build the instrument, choose which crook-length becomes permanent, and keep calling its home note "C" so players don't have to relearn every fingering they already know. A B♭ trumpet kept the length that used to need the B♭ crook. Hence: written C, sounding B♭.
Pick one of the wind or brass presets and the readout does the arithmetic for you — it locks onto the real acoustic pitch, then reports the written note that pitch corresponds to for that instrument:
Notice the trumpet and the clarinet share an interval despite being built completely differently — one's a cylindrical reed instrument, one's a conical brass instrument — because both got named after the same B♭ crook length convention, decades apart.
Concert bands tune to a single reference note before rehearsal, almost always concert B♭ — the frequency, not the written name. A B♭ trumpet or B♭ clarinet player gets there by playing their written C. An E♭ alto player has to work out a written A to land on the same concert B♭. A flute player just plays B♭, because for them there's nothing to translate. If you've ever wondered why the conductor says one note and half the room fingers something that looks completely different, that's why — everyone's aiming at the same sound wave from a different map.
Switch the tuner to Trumpet B♭ (or Clarinet B♭, or Alto Sax E♭) and play any note. The big readout is the concert pitch actually reaching the mic — that's what gets checked against the meter, because physics doesn't care what your part calls it. Underneath, a second line spells out the written name a player would see on the page for that same note. Play a long tone, watch both names update together, and the two-hundred-year-old habit stops being confusing and starts being obvious.