A half step (also called a semitone) is the distance between any note and the very next note — the smallest possible move in Western music. On a piano, it's one key to the next, whether white or black. On a guitar, it's one fret.

Examples of half steps:

• C to C#
• E to F (no black key between them — this is a natural half step)
• B to C (another natural half step)
• F# to G

A whole step (also called a tone) is two half steps. You skip one note in between.

Examples of whole steps:

• C to D (skip C#)
• E to F# (skip F)
• A to B (skip Bb)

Half steps and whole steps are the building blocks of scales. The major scale, the minor scale, every mode — they're all just specific patterns of whole steps and half steps. When someone says the major scale formula is W-W-H-W-W-W-H, this is what they mean. Don’t worry about memorizing that pattern just yet, we'll get to scales soon enough... for now that's just a clear example of why we're learning this.

Once you can see the difference between a half step and a whole step, you can build any scale from any starting note. That's the unlock.

For this lesson you don't need all of these. Feel free to move on as soon as you have a decent grasp of the concept.

• Open Music Theory — Half Steps, Whole Steps, and Accidentals — Free open textbook. Clear, illustrated, includes accidentals. Probably the only resource you actually need on this topic.
• Musical U — About Whole Steps and Half Steps — A podcast episode that covers all of this content, if that's your kind of thing.
• Wikipedia — Semitone — Technical reference. Covers chromatic vs. diatonic semitones if you want the full picture.