HMS Pinafore is a typical Gilbert & Sullivan operetta, a slightly confusing plot, which all comes right in the end and some lovely music to accompany it. I won't attempt to condense a plot summary into this space, so if you wish to know the plot, have a look here  
Cox & Box is often labelled a G&S operetta, but Gilbert was actually not a part of this one! This short operetta concerns just 3 people - Cox, Box and their landlord. It is the story of two people sharing the same flat and yet not knowing it as one is away at night and the other is away during the day. 

Some of the more famous songs from the two operettas are: