B.C. and A.D. are abbreviations historians use to mark years in history. B.C. stands for “Before Christ” and counts backward from the time people traditionally mark as the birth of Jesus Christ. A.D. stands for “Anno Domini,” Latin words meaning “in the year of our Lord,” and counts forward from that same point.

When you see a date like 500 B.C., it means 500 years before that starting point. The larger the B.C. number, the further back in time: 1000 B.C. came before 500 B.C., which might seem backward at first. Meanwhile, A.D. 500 means 500 years after that starting point, and we're now living in A.D. 2026 (though we usually just say “2026”).

This dating system became standard across much of the world because European scholars and explorers spread it globally over centuries. The pyramids of Egypt were built around 2500 B.C., ancient Rome fell around A.D. 476, and Columbus reached America in A.D. 1492.

Today, some historians prefer the terms B.C.E. (Before Common Era) and C.E. (Common Era) instead, which mark the exact same years but use religiously neutral language. Whether you see B.C./A.D. or B.C.E./C.E., they're referring to the same timeline, just with different names for the same dividing point in history.