At the moment, the Bruins’ top four defensemen appear to be on firm ground. But the third pairing? Well, that appears to be a question that still needs answering, and there could be some experimenting in the final 13 games before the playoffs.

In Monday’s practice, Charlie McAvoy took his usual spot with Matt Grzelcyk whileandHampus Lindholm was paired with Brandon Carlo in the usual second un

But with four players to fill two spots, the coaching staff went with two lefties (Mason Lohrei and Parker Wotherspoon) for one pairing and two right-handed D-men (Kevin Shattenkirk and Andrew Peeke) for the other.

“You’re trying to see if guys are comfortable playing the offside, especially when we knew today was going to be a good practice where we would be defending the rush a lot,” said Coach Jim Montgomery after Monday’s practice in Brighton. “There were three drills defending the rush, so we see guys playing on the offside. And we tell them to switch, so they get comfortable playing with each other like that. And then a little bit is just seeing what we have.”

Shattenkirk, a scratch the last two games, played on the left side in practice with Peeke, something he’s done sparingly during his lengthy career. While there was no confirmation that he will play on Tuesday against the Ottawa Senators, he was on the second power-play unit, which is usually a pretty good clue.

A veteran of 15 NHL seasons and a pro’s pro, Shattenkirk expected to be in a battle for playing time.