What is the sound of the open 2nd string?

The sound of the open 2nd string on a standard-tuned guitar is indeed a B note. In standard tuning, the strings are tuned from the lowest (thickest) string to the highest (thinnest) string as E, A, D, G, B, and E. The 2nd string specifically is tuned to B.

When you play the string without pressing down any frets, it vibrates at a frequency that corresponds to this B note, which is the musical pitch associated with the open 2nd string. Understanding the note names of the open strings is fundamental for navigation and comprehension of the fretboard, as it helps in identifying intervals, building chords, and finding notes throughout different positions on the guitar.