The K-T (Cretaceous-Tertiary) boundary is well known as the meteor impact layer that killed off the dinosaurs 66 million years ago. The reason Cretaceous is abbreviated with a “K” is because the original German word for the chalk associated with the Cretaceous is “Kreide” -- plus the “C” abbreviation was already taken by the Carboniferous time period. Anyway, geologists now go with Paleogene instead of Tertiary so it’s now the K-PG Boundary.