Correlation
How two pairs move relative to each other. +1 = in sync. -1 = inverse.
EURUSD and GBPUSD often run +0.8 to +0.9 correlated. Long both = essentially one bet, doubled. Watch correlation when stacking positions so you don't take 3x intended risk on the same theme.
Related
Learn to actually use Correlation.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.