piano scale
F Phrygian
F1G♭♭2A♭♭3B♭4C5D♭♭6E♭♭7
Minor with a flattened second — the half step right above the tonic is the whole character.
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♭WB♭WCHD♭WE♭WF
What is true of it
F Phrygian is F, G♭, A♭, B♭, C, D♭ and E♭, and back to F.
Its steps are H – W – W – W – H – W – W, where W is a whole step and H a half.
On the same tonic, F natural minor differs from it by exactly one note.
Chords made only of these notes
CdimdiminishedCm7♭5half-diminished seventhD♭majorD♭maj7major seventhE♭mminorE♭m7minor seventhFmminorFm7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.