Sources

S1 — musictheory.net: Lessons

authority: authoritative-guide
supports: Lessons 01-06 — the musical alphabet and the keyboard, sharps/flats raising and lowering by a half step, half and whole steps, interval numbers counted by letter names, the major-scale step pattern, key signatures, and how triads are stacked in thirds.
key-fact: musictheory.net's free lessons walk a beginner from note names on the staff and keyboard through accidentals, intervals, scales, key signatures, and chord (triad) construction, in the same order this course uses.

authority: encyclopedia
supports: Lesson 01 — the seven letter names A B C D E F G repeat, and notes one octave apart share the same letter name.
key-fact: "Traditionally... the seven notes of the current 'natural' A minor scale... are as follows: A, B, C, D, E, F, G." Notes an octave apart are given the same name because they are perceived as the same pitch class.

authority: encyclopedia
supports: Lesson 01 — a sharp and the flat of the next letter can name the same key on a piano (enharmonic equivalents), e.g. C-sharp and D-flat.
key-fact: "two notes... are enharmonic... when they have the same pitch but are notated differently." In twelve-tone equal temperament (the tuning of a standard piano) enharmonic notes such as C-sharp and D-flat sound identical.

authority: encyclopedia
supports: Lesson 02 — a semitone (half step) is the smallest interval between two adjacent keys; a whole tone (whole step) equals two semitones; E-F and B-C are natural half steps.
key-fact: "A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music." On a keyboard it is the distance between a key and the very next key, black or white.

authority: encyclopedia
supports: Lesson 03 — an interval is the distance between two notes; its number is counted inclusively across letter names (C up to G spans five letters, a fifth).
key-fact: "In music theory, an interval is a difference in pitch between two sounds." The interval number "is the number of letter names or staff positions (lines and spaces) it encompasses, including the positions of both notes forming the interval."

authority: encyclopedia
supports: Lesson 04 — the major scale is the step pattern whole-whole-half-whole-whole-whole-half; C major uses only the white keys.
key-fact: "The simplest major scale to write is C major, the only major scale not requiring sharps or flats." The major scale is built from the sequence of steps "whole, whole, half, whole, whole, whole, half."

authority: encyclopedia
supports: Lessons 04-05 — a key signature is the set of sharps or flats placed at the start of the music; the sharps are added in the fixed order F, C, G, D, A, E, B.
key-fact: "A key signature is a set of sharp, flat, or rarely, natural symbols placed on the staff at the beginning of a section of music." "The order of sharps is F, C, G, D, A, E, B" and the order of flats is the reverse.

authority: encyclopedia
supports: Lesson 06 — a triad is three notes (root, third, fifth) stacked in thirds; whether the lower third is major or minor makes the triad major or minor.
key-fact: "A triad is a set of three notes... that can be stacked vertically in thirds." A major triad has a major third above the root, and a minor triad has a minor third above the root.

authority: encyclopedia
supports: Lesson 05 — moving up by a fifth adds one sharp to the key signature; C major has none, G major has one (F-sharp), D major has two.
key-fact: "The circle of fifths organizes pitch classes... Starting on C and going clockwise by ascending fifths, each successive key adds one sharp" — C (0), G (1), D (2), and so on.