The Apartment by Billy Wilder and I.A.L. Diamond.
Mr. Wilder and Mr. Diamond won the 1960 Oscar for Best Original Screenplay. 
A personal note: Billy Wilder was one of the first Hollywood legends I ever saw in person, in 1994, at The Grill in Beverly Hills. He was eating lunch with Warren Beatty.

The Apartment by Billy Wilder and I.A.L. Diamond.

Mr. Wilder and Mr. Diamond won the 1960 Oscar for Best Original Screenplay. 

A personal note: Billy Wilder was one of the first Hollywood legends I ever saw in person, in 1994, at The Grill in Beverly Hills. He was eating lunch with Warren Beatty.