During the 11 seasons of the original Frasier, we got to know Niles' wife Maris, but we never got to see her. It was a running joke for years on the show, up until Niles and Maris got a divorce. But they talked about her enough that we all felt we knew her.
The idea came from the show that Frasier spun off from: Cheers. In that series, the barfly character Norm was married, but although he often referred to his wife Vera, she was never seen. We came close to meeting her once during a Thanksgiving episode, but that story involved a food fight, and when Vera walked in at the end, her face was immediately covered with pie. The idea traveled to the Frasier spinoff, but the producers wanted to avoid comparisons with its parent show, so there were plans to introduce Maris eventually from the beginning. Why didn't that happen? Because that joke, told by the right writers, went in a completely different direction. David Hyde Pierce, who played Niles, explains what the plan for Maris started out as, and how it evolved as the show became a hit, at SlashFilm.