In the section on scales, you can see how it is possible to build a scale starting on any note. The name of the note you start on is also the name of the key.

Here are a few examples:

This Major scale of C could also be said to be in the key of C. Because no sharps or flats are needed to preserve the pattern of tones and semitones, the key signature (the sign at the beginning of a piece of music which identifies the key) looks like this: