Complex sentence: Boston earned their sixth consecutive, and seventh overall NBA championship, as they continued to dominate the 1960s; with the exception of 1967, the Celtics won every NBA championship awarded in the sixties.
Split into simpler sentences: Boston earned their sixth consecutive, and seventh overall, NBA championship, as they continued to dominate the 1960s decade. Indeed, with the exception of 1967, the Celtics won every single NBA championship awarded in the sixties.