oblique motion: In musical notation, when one musical line ascends or descends, while the other continues or repeats the same note.

octave: Twelve half-steps, from one note to the next note of the same name.

octave mark: Two dots that appear on many guitars at the twelfth fret used to signify the octave.

octave method; two-frets method; two-strings method: A method of finding the same notes in different places on the fretboard.

open chords; open-position chords: Chords that contain open (non-fretted) strings.

open position: A combination of all the open strings plus the notes in the second position on a guitar.

open string: A string that is not fretted.

open-position chords; open chords: Chords that contain open (non-fretted) strings.

output jack: The insertion point for the cord that connects the guitar to an amplifier or other electronic device.