Figure 1. The hand sign for Re (pitch of D) in solfege.
Figure 2. The notes of D in different octaves on the stave, in scientific pitch notation, and on a standard piano keyboard, with frequencies shown in hertz.
Using a reference frequency of 440 Hz for the A above middle C, and calculated in equal temperament, middle D (D4) has a frequency of 293.665 Hz (see Figure 2). The 4th string of a guitar in open tuning is tuned to D3.
D may also refer to the key of D major.