piano scale
F altered
F1G♭♭2A♭♭3B♭♭♭4C♭♭5D♭♭6E♭♭7
The seventh mode of melodic minor. Every degree above the tonic that can be altered, is.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
FHG♭WA♭HB♭♭WC♭WD♭WE♭WF
What is true of it
F altered is F, G♭, A♭, B♭♭, C♭, D♭ and E♭, and back to F.
Its steps are H – W – H – W – W – W – W, where W is a whole step and H a half.
Written correctly it needs 7 alterations, B♭♭ among them. That is what the letters force, and it is also why nobody writes music in this key: the same notes spelled from the other side are easier to read.
On the same tonic, F Locrian differs from it by exactly one note.
Chords made only of these notes
D♭majorD♭augaugmentedD♭7dominant seventhE♭dimdiminishedE♭m7♭5half-diminished seventhFdimdiminishedFaugaugmentedFm7♭5half-diminished seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.