This course assumes that you have a basic knowledge of how to raise and lower pitches in standard music notation. If you need to review proper usage of accidentals, please refer to the Further Reading section under Lesson 1b.
When labeling pitches, we also need a way to refer to specific octaves or registers. We will be using the system used by the International Standard Organization (ISO). In this system, middle C is labeled as C4.
At its core, enharmonic equivalence is an easy concept: When two pitches sound the same – meaning that they share identical wavelength frequencies – but have different note names (i.e. letters), they are considered to be enharmonically equivalent. All pitches that are enharmonic equivalents create a pitch-class, so there are twelve possible pitch-classes. Every pitch has multiple enharmonic equivalents, but some are used less frequently due to the necessity for uncommon accidentals such as double-sharps and double-flats. Review the examples under Lesson 1b to make sure that you understand the less common examples. Note that all but one pitch-class has at least three enharmonic equivalents when using the five most-common accidentals: naturals, flats, sharps, double-flats, and double-sharps. (The remaining pitch-class only has two possible enharmonic equivalents without creating accidentals that exist only in theory such as triple-sharps or triple-flats.)