Sources

S1 — musictheory.net: Lessons

URL: https://www.musictheory.net/lessons

• authority: authoritative-guide
• supports: 第 1-6 课：音乐字母表与键盘、升降号按半音升降、半音与全音、按字母名数出音程度数、大调音阶公式、调号，以及三和弦按三度叠置的构成。
• key-fact: musictheory.net 的免费课程按与本课相同的顺序，把初学者从五线谱与键盘上的音名一路带到变化音、音程、音阶、调号与和弦（三和弦）构建。

S2 — Wikipedia: Musical note

URL: https://en.wikipedia.org/wiki/Musical_note

• authority: encyclopedia
• supports: 第 1 课：七个字母名 A B C D E F G 循环使用，相隔一个八度的音共享同一个字母名。
• key-fact: "Traditionally... the seven notes of the current 'natural' A minor scale... are as follows: A, B, C, D, E, F, G." 相隔八度的音被赋予相同的名字，因为它们被感知为同一个音级。

S3 — Wikipedia: Enharmonic

URL: https://en.wikipedia.org/wiki/Enharmonic

• authority: encyclopedia
• supports: 第 1 课：一个升号音和下一个字母的降号音可以指钢琴上同一个键（等音），例如升 C 和降 D。
• key-fact: "two notes... are enharmonic... when they have the same pitch but are notated differently." 在十二平均律（标准钢琴的调音）中，升 C 与降 D 这样的等音听起来完全相同。

S4 — Wikipedia: Semitone

URL: https://en.wikipedia.org/wiki/Semitone

• authority: encyclopedia
• supports: 第 2 课：半音是相邻两键之间的最小音程；全音等于两个半音；E-F 和 B-C 是天然半音。
• key-fact: "A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music." 在键盘上，它是一个键与最邻近下一个键（无论黑白）之间的距离。

S5 — Wikipedia: Interval (music)

URL: https://en.wikipedia.org/wiki/Interval_(music)

• authority: encyclopedia
• supports: 第 3 课：音程是两个音之间的距离；其度数按字母名包含两端地数（C 往上到 G 横跨五个字母，五度）。
• key-fact: "In music theory, an interval is a difference in pitch between two sounds." 音程的度数 "is the number of letter names or staff positions (lines and spaces) it encompasses, including the positions of both notes forming the interval."

S6 — Wikipedia: Major scale

URL: https://en.wikipedia.org/wiki/Major_scale

• authority: encyclopedia
• supports: 第 4 课：大调音阶是全-全-半-全-全-全-半的音级公式；C 大调只用白键。
• key-fact: "The simplest major scale to write is C major, the only major scale not requiring sharps or flats." 大调音阶由 "whole, whole, half, whole, whole, whole, half" 的步序构成。

S7 — Wikipedia: Key signature

URL: https://en.wikipedia.org/wiki/Key_signature

• authority: encyclopedia
• supports: 第 4-5 课：调号是放在乐谱开头的一组升号或降号；升号按固定顺序 F、C、G、D、A、E、B 加入。
• 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"，降号的顺序与之相反。

S8 — Wikipedia: Triad (music)

URL: https://en.wikipedia.org/wiki/Triad_(music)

• authority: encyclopedia
• supports: 第 6 课：三和弦是按三度叠置的三个音（根音、三音、五音）；底部三度是大是小，决定三和弦是大是小。
• key-fact: "A triad is a set of three notes... that can be stacked vertically in thirds." 大三和弦在根音上方是大三度，小三和弦在根音上方是小三度。

S9 — Wikipedia: Circle of fifths

URL: https://en.wikipedia.org/wiki/Circle_of_fifths

• authority: encyclopedia
• supports: 第 5 课：每往上一个五度，调号多一个升号；C 大调没有升号，G 大调一个（升 F），D 大调两个。
• key-fact: "The circle of fifths organizes pitch classes... Starting on C and going clockwise by ascending fifths, each successive key adds one sharp"，即 C（0）、G（1）、D（2），依此类推。