C is considered the main key, but some instruments are built in a different key. And you can figure how how different they are by switching the notes that the keys are named after.
The most common ones are C, like a Flute or Piano. When they play a C, it’s a C.
Some are in Bb(B-Flat), like a Trumpet or Clarinet. When they play a C, it’s actually a Bb.
Some are in Eb(E-Flat) the main ones here are the Alto Saxophone or Baritone Saxophone. When they play a C, it’s actually an Eb.
The last one you’ll really see is F, which is mostly for French Horns(the super curly one that points backwards). When they play a C, it’s actually an F
The main points of transposition is to be able to have a common ground to work from, without having to redesign every instrument to be in C, and so some instruments have patterns so you can learn more easily
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