tre corde
An instruction to release the una corda pedal, returning to full volume.
In Depth
Tre corde means three strings and instructs the pianist to release the una corda (soft) pedal, allowing the hammers to strike all three strings for each note. This restores the piano's full, normal tone after a passage played with the muted, softer quality of the una corda pedal.
The marking only makes sense in the context of a preceding una corda instruction. When the una corda pedal is depressed on a grand piano, the entire hammer mechanism shifts so that the hammers strike only two of the three strings (or one of two in the bass), reducing both the volume and changing the timbre. Tre corde reverses this shift.
On Beethoven's pianos, the una corda pedal shifted the hammers far enough to strike only one string — modern pianos shift less, so the contrast between una corda and tre corde is more subtle today.