LONDON — London’s relentless building boom has dug up another chunk of the city’s history — one with a surprise for scholars of Shakespearean theater.

Archaeologists are excavating the remains of the Curtain, a 16th-century playhouse where some of the Bard’s plays were first staged, before a new apartment tower sprouts on the site. Unexpectedly, the dig has revealed that the venue wasn’t round, like most Elizabethan playhouses. It was rectangular.

That came as a surprise, because the best-known fact about the Curtain is that Shakespeare’s “Henry V” was first staged here — and the play’s prologue refers to the building as “this wooden O.”

“This is palpably not a circle,” Julian Bowsher, an expert on Elizabethan theaters, said during a tour of the site Tuesday.

The discovery has made Bowsher rethink some of his ideas about Tudor playhouses. He suspects that the Curtain — unlike the more famous Globe and Rose theaters — wasn’t built from scratch, but converted from an existing building.

“Out of the nine playhouses that we know in Tudor London, there are only two that have no reference to any construction,” he said — including the Curtain. “It’s beginning to make sense now.”