The bass cleft is used for the lower what in piano music?

The bass clef, or F clef, is used on the staff for instruments playing mostly below middle C (C4). This keeps most of the notes in the bass clef staff, making them easier to write and read.

See the Wikipedia article called "Scientific Pitch Notation" for an illustration of 10 octaves of the note C, using bass and treble clefs (F and G clefs): http:/en.wikipedia.org/wiki/Scientific_pitch_notation