Music Theory Reference

Keys, scales, diatonic chords, secondary dominants, borrowed chords, and the chord library.

Nitesong is built with music theory at its core. Every song you create has a musical key, and that key drives the chord suggestions, scale references, and sidebar tools throughout the editor.

This page covers how Nitesong represents keys, scales, chords, and harmonic relationships — and how all of that surfaces in the UI while you write.

Keys & Scales

Supported keys

Nitesong supports 25 musical keys — 13 major and 12 minor:

Every song must have a key assigned at creation time. The key you choose drives everything else described on this page: which chords appear in the "In Key" category, which secondary dominants are computed, which borrowed chords are offered, and what notes are highlighted on the sidebar fretboard and keyboard tools.

How keys work in practice

When you set a song's key (either at creation or by clicking the key badge in the song header), Nitesong automatically derives:

• The scale — the 7 notes that belong to the key (e.g., C major gives C, D, E, F, G, A, B)
• The diatonic triads — the 7 chords naturally built from the scale
• The diatonic seventh chords — extended versions of those same 7 chords
• The relative key — the related major or minor key that shares the same notes (e.g., C major's relative is A minor)

Song Key Panel

The Song Key Panel appears in the right column of the song header, giving you an at-a-glance reference for the current key.

What it shows

The panel has three parts:

Header row — displays the key label (e.g., "C major"), the relative key (e.g., "rel. A minor"), and the song's BPM.

Scale grid — a 7-column grid showing:

• The scale degree number (1 through 7)
• The note name below each number (e.g., C, D, E, F, G, A, B for C major)

Diatonic chords grid — another 7-column grid showing:

• The Roman numeral (e.g., I, ii, iii, IV, V, vi, vii°)
• The triad symbol (e.g., C, Dm, Em, F, G, Am, Bdim)
• The seventh chord symbol below (e.g., Cmaj7, Dm7, Em7, Fmaj7, G7, Am7, Bm7b5)

Diatonic Chords

For any given key, Nitesong knows the 7 diatonic chords — the chords that naturally belong to that key. These are the chords you will reach for most often when writing in a key.

Major key example (C major)

Minor key example (A minor)

Roman numeral conventions

Nitesong follows standard music theory notation:

• Uppercase Roman numerals (I, IV, V) indicate major chords
• Lowercase Roman numerals (ii, iii, vi) indicate minor chords
• The degree symbol (vii°) indicates a diminished chord
• Flat signs (bIII, bVI, bVII) appear when a chord root is lowered from its natural position

The "In Key" category in the chord picker includes not just the basic triads and sevenths but also sus2 and sus4 variants for the I and V degrees. For example, the key of C major includes C sus2, C sus4, G sus2, and G sus4 alongside the standard 7 diatonic chords. Chords are sorted by scale degree.

Secondary Dominants

Secondary dominants add tension and motion to your progressions by briefly tonicizing a diatonic chord. A secondary dominant is the V7 (dominant seventh) chord of a target diatonic chord — it creates a momentary pull toward that target before resolving.

Example in C major

Notice there is no V7/I — you would not tonicize the tonic itself, since you are already there.

Where they appear

In the chord picker, secondary dominants appear under the "Secondary Dominants" heading, between the "In Key" and "Borrowed" categories. This makes them easy to find when you want to add some harmonic color without stepping too far outside the key.

Borrowed Chords

Borrowed chords (also called modal interchange) come from the parallel key — the major or minor key that shares the same root note. If you are writing in C major, borrowed chords come from C minor. If you are writing in A minor, they come from A major.

Borrowed chords are a classic way to add emotional depth. A bVI chord in a major key (e.g., Ab major in the key of C) creates a familiar, bittersweet sound heard in countless songs.

Example in C major

Borrowed from C minor (natural minor):

Where they appear

In the chord picker, borrowed chords appear under the "Borrowed" heading, after "Secondary Dominants" and before "All Chords".

The 122 Chord Library

Nitesong ships with 122 predefined chords covering 8 qualities across all 12 chromatic root notes.

Chord qualities

Additional enharmonic variants bring the total to 122.

Chord picker categories

When you open the chord picker on an element, chords are organized into four groups:

1. In Key — diatonic chords sorted by scale degree, each showing its Roman numeral label.
2. Secondary Dominants — the V7 of each diatonic chord, for adding tension and motion.
3. Borrowed — chords from the parallel key that add color through modal interchange.
4. All Chords — everything else, for when you want something completely outside the key.

Nitesong automatically computes the secondary dominants and borrowed chords for your key and organizes the full chord library into these four categories.

How It All Fits Together

Here is the flow from setting a key to seeing chords in the picker:

1. You set the key on your song (e.g., "C major") via the key badge or creation form.
2. The Song Key Panel renders the scale degrees, triads, and seventh chords in the header for quick reference.
3. When you open the chord picker, chords are automatically organized into four smart categories — In Key, Secondary Dominants, Borrowed, and All Chords — based on their harmonic relationship to your song's key.
4. Sidebar tools (fretboard, keyboard, circle of fifths) highlight the scale notes on their respective visualizations.

This design means you get rich music theory context without needing to think about it — the theory is always there when you want it, organized by how useful each chord is likely to be for the key you are writing in.