President Donald Trump took aim at a set of precautionary pardons Joe Biden issued in the remaining hours of his presidency, saying they are invalid due his use of an autopen.

Trump posted on Truth Social after midnight on Monday and alleged Biden had used the mechanical device when he issued a last-minute set of pardons before leaving office for those he believed to be political enemies of Trump.

“The “Pardons” that Sleepy Joe Biden gave to the Unselect Committee of Political Thugs, and many others, are hereby declared VOID, VACANT, AND OF NO FURTHER FORCE OR EFFECT, because of the fact that they were done by Autopen,” Trump posted.

He added: “In other words, Joe Biden did not sign them but, more importantly, he did not know anything about them!”

But what is an autopen? Is the signature legal, what does it look like and what does it mean for Biden’s pardons? Here’s what to know.

What is an autopen?

An autopen is a device used to replicate handwritten signatures without direct human involvement. It was first patented as a “polygraph” machine in 1803 and has since evolved into digital form. Many public figures have been known to use autopens for decades, including Jon Bon Jovi and Bob Dylan.

The use of the autopen has otherwise been uncontroversial over the years and never has been challenged or tested in court, with use of some form of signature copying devices dating back to the 1800s.

What does an autopen look like?

There are several brands of digital autopens, but Ghostwriter is believed to be the nation’s best-known and used most often, according to their website. Here is an example of how their T Series works:

Is the autopen legal?

A 2005 legal study commissioned by former President George W. Bush tasked the U.S. Department of Justice to look into the auto pen’s constitutionality. While Bush never used it himself, the 29-page opinion called it legal.

Who did Joe Biden pardon?

In the final hours of his presidency, Joe Biden pardoned:

• Anthony Fauci, former director of the National Institute of Allergy and Infectious Diseases
• Retired Gen. Mark Milley, former chairman of the Joint Chiefs of Staff
• Lawmakers and staffers from the House select committee that investigated the Jan. 6, 2021, Capitol attack. Those lawmakers included GOP Reps. Liz Cheney of Wyoming, Adam Kinzinger of Illinois, and Sen. Adam Schiff of California
• His brother, James Biden, and his wife, Sara Jones Biden
• His sister, Valerie Biden Owens, and her husband, John Owens
• Another brother, Francis Biden

What presidents used autopen?

It is believed the autopen machine was first used by a president in 2011 when President Barack Obama signed the expiring Patriot Act from France, where he was attending the G8 summit. He used it two other times during his presidency.

The first autopen, called the polygraph, was patented in 1803. It was later used by Thomas Jefferson, who called it the “finest invention of the present age,” according to Shapell Manuscript Foundation, an educational organization dedicated to research and collection of manuscripts and historical documents.

It is believed Harry Truman made use of the device; Gerald Ford was open about utilizing it; and Lyndon B. Johnson appeared on the cover of “The National Enquirer” sitting with the device while in the White House, according to Shapell.

John F. Kennedy was so dependent on the autopen, according to Shapell, that he was the subject of a book entitled “The Robot That Helped to Make a President.”

Lori Comstock is a New Jersey-based journalist with the Mid-Atlantic Connect Team.