How does periodization historians study the past?

It is known as periodization to the field of social sciences that tries to divide history in different periods that have common traits among themselves, important enough to make them qualitatively different from other periods.

In history, the broader periodization gives periods called ages, whereas the so-called epochs designate shorter divisions. Periodization facilitate the study of history, by encompassing interrelated events.

rosevelt. 1973. "slaves everywhere"

“i think it is very plausible that if there are five votes to end the death penalty, it could be with the people who are on the court right now,” - smith

so basically it is not clearly and consistently imposed