Tap any button in the Alterations row — ♭5, ♯5, ♭9, ♯9, ♯11, or ♭13. Fretscape either alters the matching chord tone or, for the upper alterations, adds the altered tone as its own colour note. The chord name picks up the alteration.

What each alteration does

• ♭5 flattens the 5th. On C7 this gives C7♭5, with notes C E G♭ B♭ instead of C E G B♭.
• ♯5 sharpens the 5th. On Cm9 this gives Cm9♯5, with notes C E♭ G♯ B♭ D.
• ♭9 flattens the 9th — so on C7 you get C7♭9.
• ♯9 sharpens the 9th. On a C7 this gives the classic "Hendrix chord", C7♯9.
• ♯11 sharpens the 11th. Most commonly used on a major 7 chord to get Cmaj7♯11.
• ♭13 flattens the 13th — a dark, jazzy colour on a dominant 7.

Each alteration shows as an amber token in the Building row, and several can stack — C7♯5♯9 is a real chord, and so is C7♭9♭13.

A note about extensions

♭9, ♯9, ♯11, and ♭13 are upper-extension alterations. They're most commonly used on chords that already have a 7th — especially dominant 7 chords. Tap one on a plain triad and Fretscape adds it as a stacked tone (so ♭9 on C gives Cadd♭9); tap one on a chord that already includes the matching extension and it alters that extension instead.

Forcing the add form for b9, #9, #11, and b13

If the parent extension is already in your chord, a tap on the alteration normally alters that extension (C9 + ♭9 → C9(♭9)). If you specifically want the stacked-tone form even when the parent is present, long-press the alteration button (or right-click on a computer) to open an add option in the menu. That gives you, for example, C9(add♭9) instead of C9(♭9).

The other two alterations — b5 and #5 — work as straight tone-replacements and don't have an add form.

See How do I add or leave a note out of a chord? for the full set of modes.

Removing an alteration

Each alteration shows as its own amber token in the Building row with a small × for removal. Tap the × to drop the alteration on its own — the rest of the chord stays as you built it, and the voicing list rebuilds.