Identify the note at the 12th fret of the 4th (D) string.

The note at the 12th fret of the 4th string, which is tuned to D, is indeed D. This is because the 12th fret represents an octave of the open string note. When you press down at the 12th fret, you are raising the pitch of the open string by a full octave. In the case of the 4th string, which is tuned to D, the note played at the 12th fret is also D, making it a fundamental reference point on the fretboard. This concept of octave relationships is essential for understanding the structure of the guitar fretboard and how notes are arranged in relation to one another.

Knowing that the open 4th string is D, any fret along that string allows players to understand the note at that position in relation to the open string, emphasizing the importance of recognizing this pattern across all strings on the guitar.