Cardinal decades are decades based on the plain cardinal numbers. This groups years together based on what the numbers start with. 1970–1979 were "the '70s."

Cardinal decades do not exactly align with ordinal decades. This is because "the 0s" only had nine years due to there not being a year zero. This means any year ending in "10" will be part of the previous ordinal decade and the next cardinal decade.

Cardinal decades are still just arbitrary delineations of time, like most of our calendar systems, but they are more practical than ordinal decades at least. Most people intuitively think of decades in the cardinal sense. We typically will refer to "the roaring '20s," "the '90s," or even "the aughts." I have literally never heard someone say "the 193rd decade."