President Donald Trump has claimed that pardons signed by his predecessor Joe Biden are invalid. His reasoning: They were allegedly signed with an autopen, a signing machine used by the U.S government for decades.

The use of the autopen, a device that reproduces signatures and is ubiquitous in government and business, is ordinarily uncontroversial. There is no power to undo a pardon in the Constitution or case law.

Donald Trump says Joe Biden's last-minutes pardons via autopen are null and void. But what is an autopen? And what does it mean for pardons?