But, if you know the "locrian mode" you are refering to a PATTERN on the fretboard. That's a hard habit to break...because a pattern does not constitute a mode, or a name. The chord you are playing the notes over tells you the root, which in turn gives you the name.
I'm going to post a series of images showing you all the boxes of the Diatonic Major scales. There will be 7 INDENTICAL patterns...but with different names. This will show the one PATTERN is EVERY MODE.
The final image will also contain a COMMON drawing guitarists get from their teacher. These drawing will attach a NAME to a PATTERN...this is where most of the confusion about modes for guitarist starts, and it avalanches fast from there.
So, try and realize that one PATTERN is ALL of the modes, not a individual one, and ANY note in the pattern could be a Root depending on what chord it's played over.