Like theatre in the US, you can wear anything you want. Pants, jeans, suits, dressy stuff - all goes. Locals tend to dress up more - suits and the like - particularly if it's Saturday night. However, you wouldn't feel out of place wearing more casual gear.

Funny story: I travel light. One carry on and that's it. So I have basics only. Bought tickets for the Opera in Madrid and turned out it was an opening night with King and Queen, et al. The best I could come up with was black cotton pants (which I have been known to use as peja bottoms or in dance class), a cotton top with scarf, black sox and Birkies (I took them over my tennies). Paraded through the lobby with press and first nighters, all in full dress. However, once I found my seat in the upper, upper balcony, I was ok. The students were all about, in their jeans and tennies. And I was at home!

Regardless of what you wear, you WILL enjoy the play.

Jo, still in Sunnyvale