Expert (talent)

Essential. Become an expert in one skill or tool in which you are already proficient. Being an expert lets you add a bonus die to applicable ability checks. The size of the die depends on your proficiency bonus. A +2 bonus grants a d4, +3 grants d6, +4 grants d8, +5 grants d10, and +6 grants d12.

Being an expert in a subject also counts as having "expertise" in a subject. The effects can't combine.