Chord

A chord was a set of notes played in sequence, with these sequences forming a series and becoming music. (PROSE: Love and War) A single chord could also be repeated in a rhythm to form music. (PROSE: Transit)

Music
Chord changes often signalled shifts in styles of music, or the transition between lyrics in songs. (PROSE: Happy Endings) On instruments like the guitar chord changes could be achieved by moving fingers along the neck of the guitar. (PROSE: The Left-Handed Hummingbird) The playing of certain music on an organ could produce throbbing chords. (PROSE: Doctor Who and the Pyramids of Mars)

Specific musicians' abilities
Danny Pain of Plasticine only knew three chords. (PROSE: No Future)

Fitz Kreiner would sometimes play through chord progressions on a battered Fender Telecaster plugged into an amplifier (itself plugged into the TARDIS console). (PROSE: The Slow Empire)

Language
To "strike a chord" is to have something seem similar, or to have something said or implied that would remind an individual of something. It could also be the providing of inspiration or the prompting of an association of ideas presented to the individual to allow them to draw new or not thought of conclusions. (TV: The Christmas Invasion, PROSE: The Dying Days, Interference - Book Two)