Pitch – the highness or lowness of a given musical sound
Note – a written symbol for a given pitch

Our standard Western musical universe has a basic set of 7 pitches. We name them with letters of the alphabet: A B C D E F G. Here A is the lowest note and G is the highest.

Step – the distance between to adjacent letter names (A to B)
Leap – the distance between nonadjacent letter names (A to C, F to B)

If we want to go higher than G, we start our letter names over again (A B C D E F G A B C etc.) For reasons that have to do with the physics of sound, we name these pitches eight note names apart the same letter (this distance is called an octave).

This gives us 7 alphabet letters in total. A through G repeating in both directions.

We also modify this alphabet to name the 5 other pitches. To do this we add one of two other words to our letters: flat and sharp. We can have ‘A flat’ and ‘G sharp’ (‘A flat’ is lower than ‘A’ and ‘G sharp’ is higher than ‘G’). This gives us a total of 21 note names to cover the 12 pitches in our standard tuning system.