E major key

Key Signature: The key signature is a set of sharps or flats placed at the beginning of a staff to indicate which notes are altered throughout a piece in order to maintain consistency within a particular key.

Relative Minor: Every major key has a relative minor key that shares the same key signature. In this case, C# minor is the relative minor of E major.

Dominant Chord: In the context of E major, the dominant chord would be B major. The dominant chord has an important role in creating tension and leading back to the tonic chord (E).