Sen. Mitt Romney, R-Utah, signs the oath book after being sworn in Thursday for the impeachment trial of President Trump at the U.S. Capitol in Washington. Senate Television via AP

WASHINGTON — Senate impeachment trials are so important that a simple, spoken oath is not enough. Senators also must sign their names on the creamy pages of a dark-covered oath book.

That’s why the swearing-in process at Thursday’s Senate impeachment trial against President Trump was a two-step process.

After taking his own verbal oath, Chief Justice John Roberts asked senators to rise and raise their right hands and swear to do “impartial justice” during the trial. The clerk then called each senator by name in alphabetical order. They lined up along the Republican side of the chamber for their turn to sign the oath book on a table in the well of the Senate.

One by one, they stepped forward, accepted a pen and signed the pages as the Senate ground through its transformation into a court of impeachment. The senators present for the last presidential impeachment trial – in 1999 against President Bill Clinton – did likewise.

Signing the oath book is a way of conveying the gravity of presidential and judicial impeachment trials, which are rare. All impeachment trials since 1986 have involved an oath book, according to the Senate historian’s office.

That year, the Senate conducted its first impeachment trial in 50 years, and the first ever to be televised. An oath book was introduced that the senators sign themselves, mirroring a practice in place since the 1860s by which senators mark their new terms by signing a book.

The book is stored between trials at the National Archives, according to the Senate historian.