I wanted to annotate OP's awesome work, because this really is such a useful reference for people getting into music theory. Great work OP! As a music theory 'cheat sheet' this works really well, but if this is all really new to you then it definitely requires a bit of annotation. The rows running across the top explain scale degrees . It's important in music theory to use relative terms like this, rather than use absolute note pitches. Relative terms can be applied to any key, so it's much more flexible to think that way rather than pigeonholing yourself to a single key. Each degree of the scale can also be referred to by its harmonic function , which are listed directly below that on the middle of the page. In musical shorthand, a capital Roman numeral denotes a major chord, and a lowercase Roman numeral denotes a minor chord. Note that this only shows the functions for a major key, the functions in minor keys are named the same but follow a different pattern of which degrees are major/minor. To the left are the different kinds of scales , and "h" refers to a half step (e.g. moving from a C up to a C#). A particular scale will always follow the same pattern of 'steps' - that is what makes it a scale, not the notes but the relative distance between each note. Below that is a list of [modes]( https://en.wikipedia.org/wiki/Mode_(music) ). Modes are most easily visualised as a scale on a piano from a note to the same note an octave above or below, but using only the white keys. The Dorian mode, for example, is not the same as a D major scale, but a sequence of all the white keys from D to D. Therefore the pattern of 'steps' is different for each mode, meaning each mode has different melodic characteristics. Modes are an incredibly important concept to understand in music theory. To the right of that are parallel modes , which are explained in the text in parentheses - modes and scales that share the same root note. To the right is something instantly recognisable to anyone who has studied any music theory - the circle of fifths . It's an incredibly useful graph that shows the relationship between all 12 tones of the chromatic scale. People have written entire books on the circle of fifths alone, if you can take anything away from OP's resource then this should probably be it. There's too much to explain in detail, but basically the red numbers are the keys (start at C in a clockwise direction and notice that each key goes up by a fifth, and each time you go up by a fifth you gain a sharp in the key signature or lose a flat). The number in the grey ring indicates how many sharps or flats there are in that key signature. The green number indicates the relative minor key. B/Cb and Db/C# are enharmonic - essentially the same notes, but expressed differently. To the right of the circle of fifths, that big list shows the degrees of the scale that are present in each chord . A minor7 chord is the 1st, minor 3rd, 5th, and minor 7th degrees (e.g. Cm7 is C, Eb, G, Bb). And so on. Going back to the left side, the "common progressions" shows common chord progressions in music. That is, chords that are commonly played in a particular sequence. Knowing these is helpful. To the right of that shows modal interchanges, also known as "borrowed chords" . This basically means using chords that are outside of the key you're in. Borrowed chords sound colourful because they borrow notes from different modes. It's easier to think of these as key changes that aren't actually key changes - you're suggesting notes from other modes, without actually switching to another mode. At the bottom are chord substitutions . Chord substitutions are a great way to make your music sound more dynamic and less samey - what you're doing is replacing a chord with another chord that serves the same function. These are used a lot in jazz music and they're great for subverting the expectations of the listener. Honestly, the best way to learn how to use these is to understand the theory and experiment with them yourself. The explanations that OP gives for each one are pretty helpful.