L'istesso tempo
A direction that the beat stays the same speed despite a change of time signature.
What it means
L'istesso tempo (Italian for "the same tempo") is a direction usually placed after a change of time signature. It indicates that the basic beat, not the note value, stays constant: if the metre changes from 3/4 to 6/8, without this marking it would be unclear whether the eighth note or the quarter note now carries the pulse.
Musical use
The marking appears mainly in music with frequent metre changes, such as works by Bartók, or in 19th-century opera when a recitative moves into an arioso. L'istesso tempo is distinct from a plain tempo marking such as Allegro: it always refers back to a preceding passage and preserves its underlying pulse rather than introducing a new, independent tempo.
Performance practice
Carry the smallest common note-value unit through the metre change instead of guessing a new tempo. The simplest check is to set a metronome to the previous beat and keep it running across the change.