The last white key of 3 consecutive black keyboards on the piano.

The si in treble clef is the last white key of 3 consecutive black keyboards on the piano.

si is the 7th note. There are 12 types of notes, and there are only these 12 notes in music. Among them, 7 commonly used notes are sung in order: Do, Re, Mi, Fa, Sol, La, Si. The number 1234567 in the simplified musical notation corresponds to Do, Re, Mi, Fa, Sol, La, and Si.