A flat five chord is composed using a root/1st, major 3rd and diminished 5th (or R/1-3-♭5) intervals played simultaneously. Generally written as C♭5, C♭5 or C♭5, it is good practice to simply use an uppercase C followed by a super scripted ♭5 to represent it in writing (ie: C♭5).
You will more often come across a ♭5 chord written as either a ♯11 (which is equivalent to a ♭5 an octave higher) or as a power chord with the 3rd omitted.
Flat five chord profile
|Intervals||root/1st, major 3rd, diminished 5th or R/1-3-♭5|
|Common names (examples in C)||C♭5, C♭5, C♭5|
|D Flat five chord voicing #2|