In the Genesis flood account, one pair of each unclean animal entered Noah’s Ark.
Genesis 7:2 distinguishes between clean and unclean animals. Noah is instructed to take seven pairs of clean animals and one pair of animals that were not classified as clean. Genesis 7:8–9 then summarizes the animals entering the ark two by two, male and female.
The clean-animal instruction is connected to later sacrifice, because Genesis says Noah built an altar after leaving the ark and offered some clean animals and birds. This is why the text gives different quantities rather than describing every animal as entering in exactly one pair.
Readers sometimes remember only the phrase “two by two,” which appears in the narrative’s summary. The more detailed instruction in Genesis 7:2 provides the distinction: seven pairs for clean animals and one pair for unclean animals.