The human ear perceives a musical tone
that is twice the pitch of another tone
to be the same note.
The second tone, which is the same note
as the first tone (tonic)
even though it is a higher frequency, is
said to be an octave
above the lower tone, &
a tone which is half the frequency
of another tone
is an octave lower in pitch.

A tone could be any frequency, but only a
few tones are used in most music systems.
In most systems of music, the notes are selected
from the range of possible frequencies
between octaves by a system
of intervals.
In classical music, the range between octaves is
divided into a series of 12 tones, which are
said to be a half-step apart.
The method of finding these 12 notes is based on
a system of tuning.
Many scales are comprised of notes that are
found on this 12-note or chromatic scale in
various combinations and patterns.

The major scale is a series of 8 notes at certain
intervals of the 13-note chromatic scale.
The notes of the major scale
are at distances of whole and half-steps
from one-another, a whole step being two half steps.
Each note of the chromatic scale can
be named according to its distance from
the (prime) root note
and closest note of the major scale.
This system of naming the intervals using the
major scale is also used to describe
distances between notes in chords & other scales.