Many operas such as Mozart’s Idomeneo used to be written for a part played by a male soprano. Since there are no male sopranos performing now (I’ll leave it to the reader to figure out what used to be done to some unfortunate young boys to make them male sopranos), today’s performances use one of two options. One is to cast the part as a male tenor who sings the part one octave lower than how the part was written or they can use a female soprano who play the male part.

(The Great Courses – The Operas of Mozart)