Middle C, designated C4 in scientific pitch notation and MIDI note 60, is the centermost of the eight C's on a standard 88-key piano keyboard and has a frequency of approximately 261.625565 Hz with standard pitch (A=440Hz) tuning. It serves as an approximate border between notes played on the piano with the left hand and notes played with the right. Although middle C is not in the middle of the range of most other instruments, almost all commonly used instruments have the capability to play it.
In standard music notation middle C is written on the first ledger line below the treble staff and on the first ledger line above the bass staff. It is also called C4 because it is in the fourth C from the left on a piano keyboard.