Middle C
Middle C is the C nearest the middle of the piano keyboard. It is the note from which beginners start reading music, and in scientific pitch notation it is called C4.
To find it, look for the group of two black keys closest to the middle of the piano: the white key just to its left is middle C. On a full 88-key piano it is the fourth C from the left, and in scientific pitch notation it is called C4.
On the staff, middle C fits neither the treble nor the bass staff, so it is written on a short ledger line of its own: one ledger line below the treble staff, or one ledger line above the bass staff. It is the same note and the same key, written in two ways.
On the grand staff middle C lies between the two staves, a bridge between the right-hand and left-hand notes. Going up from it are D, E, F, G (the right hand); going down are B, A, G, F (the left hand).
What it sounds like
Middle C sits in the middle register, neither low nor high. Written either way, it sounds exactly the same.
Where it is taught: the studio’s interactive textbook, chapter 1, “The keyboard and your first notes” and chapter 4, “The left hand and the bass clef”.