If your improvising over the dominant chord in the key it comes from the locrian mode would be useful. In other words in the key of C it will work over the G7 chord adding a 9th tone to the 7th.
You can of course use it to improvise over the minor 7th flat 5 chord it came from but you don’t see them too often.
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Guitar Scales Method
B Locrian Notation/Tab
Here’s a sample of how it sounds played against a Bm7♭5
The B Locrian played against a B minor 7th flat 5 chord
The Locrian List
The Locrian Formula
Here you go 1-♭2-♭3-4-♭5-♭6-♭7
For me this would be a hard one to remember this way I just think of the last note of the major scale.
I hope you found this page useful.