Key signature = what the page assumes

A key signature is a writing shortcut: it pre-loads certain sharps/flats so you don't have to mark them every time. It usually reflects a diatonic baseline (the default 7-note collection), but it does not guarantee the music stays diatonic.

Tonic = where "home" is

The tonic is the pitch that functions as home—the note melodies resolve to and the pitch other notes feel "measured against."

Example 1: Minor key with harmonic minor behavior

Take a piece notated in a minor key with its normal key signature. In real repertoire you'll often see one note repeatedly raised with an accidental right before cadences to create a stronger pull back to the tonic.

• On the page: same minor key signature throughout
• In the music: frequent raised leading-tone accidentals (harmonic-minor effect)

Key signature gives the default diatonic set; harmonic minor is "default + a consistent raised note," written with accidentals.

Example 2: Same key signature, different tonic → different mode

Two melodies can use the same key signature (same default sharps/flats) yet sound like different worlds if they resolve to different "home" notes.

• Melody #1 cadences to the "major home" the key signature suggests → feels like major
• Melody #2 uses the same written notes but cadences to a different home → now it feels like a mode (e.g., Dorian/Mixolydian/etc.)

What changes the sound is not the signature—it's the tonal center (the tonic).

Example 3: Non-diatonic modes

Some commonly used modes/scales aren't neatly covered by a standard key signature. Practical strategies:

1. Use the closest diatonic key signature, then mark altered tones as accidentals. You'll see the same accidentals recurring: those are the "mode flavor notes."
2. Use no key signature (common in jazz/pop and film cues) when the collection is highly chromatic or shifts rapidly.

Summary

• Key signature = default staff spelling shortcut (diatonic baseline)
• Tonic = the pitch that functions as home
• Scale/mode choice = the actual pitch-set used around that tonic, often requiring accidentals beyond the key signature