sus iniciales

their initials

→ su
phrase Rare #42,962
their initials Used for the first letters of a person's name. Depending on context, it can also mean 'your initials' when speaking politely to one person or to several people.
SP: Firmen aquí, sus iniciales aquí y aquí.
EN: Sign here, your initials here and here.

Sus iniciales debajo de la mía, y firme de la segunda a la última página.

His initials below mine, and signed from the second to the last page.

— Mommy (2014)

Firmen aquí, sus iniciales aquí y aquí.

Sign here, and initial here and here.

— Moonrise Kingdom (2012)

Tom talló sus iniciales en un gran roble frente al colegio.

Tom carved his initials into a large oak tree in front of the school.

La pareja grabó sus iniciales en el roble.

The couple carved their initials into the oak tree.

Y aquí sus iniciales.

And here are his initials.

— El manantial de las colinas (1986)