As military members and elected officials gathered at the State House Tuesday to honor Marines who served in the Battle of Iwo Jima, neither Gov. Maura Healey nor Lt. Gov. Kim Driscoll was on hand. Instead, sitting quietly to the side (until it was his turn to offer remarks), was the man who steps in when both Healey and Driscoll — and their predecessors for the last three decades — are away: Secretary of State Bill Galvin.

Healey and Driscoll are both out of state during the February school vacation week, Healey in Florida until Thursday and Driscoll in Georgia until Sunday. When both leave Massachusetts, an acting governor is in charge.

Under the Massachusetts Constitution, when both the governor’s and lieutenant governor’s office are vacant — “by reason of death, absence from the commonwealth, or otherwise” — it’s up to the secretary of state to fulfill duties of both.

In this case, that means Galvin, a Brighton Democrat who’s taken on the acting governor role many times since he was first elected secretary in 1994 — so many times, in fact, that a spokesperson said he’s likely lost count.

If Healey, Driscoll and Galvin are all out of state (or if one of them leaves office for another reason), the line of succession moves across the other statewide elected officers. In order, it’s: Attorney General Andrea Campbell, Treasurer Deb Goldberg and Auditor Diana DiZoglio. The state constitution stops there, specifying no further acting governors after the auditor.

During a vacancy of the two top offices, the constitution gives the acting governor “full power and authority to do and execute all and every such acts, matters and things as the governor or the lieutenant-governor might or could lawfully do or execute, if they, or either of them, were personally present.”

Galvin over the years has signed bills while serving as acting governor, and if a natural disaster or other emergency breaks out while he is in charge, he’ll need to step in for the response.

He’s also filed bills at times. The one "that brought him the most notoriety" was a 1998 bill that sought to force more financial transparency from HMOs, according to a 2003 New York Times profile that said he’s held the acting governor role more times than anyone else in Massachusetts history.

In most cases, though, an acting governor taking the reins is a brief, behind-the-scenes occurrence, involving close communication with the traveling governor and little impact on the day-to-day for most Massachusetts residents.

Typically, the governor and lieutenant governor are not both away from Massachusetts for extended periods of time. The line of succession takes on more significance in the instances when the state is without a lieutenant governor — for instance, after Lt. Gov. Tim Murray resigned in 2013, Galvin regularly had to step in when then-Gov. Deval Patrick was away.

Governor is not the only office whose duties can shift with travel plans. In Boston, if the mayor is away from the city or otherwise unable to perform their duties, the City Council president becomes acting mayor.