Other
Alien Travel Guide: Elements of Music the Basics
Fine site created for Canadian schools devoted to basic elements of music, including pitch, acoustics, time signatures, keys, and rhythm.
OpenStax
Open Stax: Catherine Schmidt Jones: Pitch Sharp, Flat and Natural Notes
Learn about the meanings and uses of pitch and accidentals. Including sharp, flat, natural and double sharps and flats, this lesson explains why they are necessary.
Other
Vocalist.org: Perfect Pitch
This brief essay discusses the writer's quest for perfect pitch even though he was not born with it.