Core rule
Lowering the critical-hit threshold
Lowering the required die result increases the set of rolls that can produce a critical hit.
Reducing the threshold from 20 to 19 allows natural rolls of 19 or 20 to qualify. Reaching 19 after adding an attack bonus is different.
Check which effects stack and which attacks they apply to. Wearing an item does not guarantee its benefit when its conditions are unmet.