In British parliament, prorogation ends one legislative year—known as a session—so another can begin. Simple, right? Not exactly. Steeped in tradition and soaked with political potential, prorogation has a history that goes back centuries. And though it has seldom been used as a tricky tactic, closing a parliamentary session is a tempting tool for British politicians eager to shut the door on pending legislation they don’t like.

The most recent one to close parliament for political gain is Conservative prime minister Boris Johnson. He plans to prorogate the longest parliamentary session in over 400 years, forcing legislators into a five-week-long break that dramatically shortens the amount of time for debate on the UK’s impending exit from the European Union.