Ionian #5 (Augmented Major) is the major scale with a raised 5th. It sounds bright but unstable - the augmented 5th creates a floating, unresolved quality. Used in modern jazz and progressive contexts.
Scale Formula
1 2 3 4 #5 6 7 - major with raised 5th. Semitone pattern: W W H WH H W H
Fretboard (C Ionian #5)
Toggle to interval view to see the #5. Everything else is natural major - only the 5th is raised, but that single alteration transforms the stability of the mode.
Common Chords
Augmented triad (1 3 #5), Major 7th#5 (1 3 #5 7).
Diatonic Harmony
Improvisation Context
Play over major 7th#5 chords. Useful when a major chord has an augmented quality, such as the bIII chord in harmonic minor harmony.
Practice
Play C Ionian #5 over a Cmaj7#5 chord. Compare it with regular Ionian - the only difference is G# vs G. Notice how the #5 creates an augmented triad (C-E-G#) that wants to resolve. Try alternating between Ionian #5 and Lydian Augmented (from melodic minor) over the same chord to hear the difference that the #4 makes.