Define the term monogram. why are they used by artists?

a motif can be 2 or more letters, like someones initials or signature, usually interwoven or otherwise combined in a decorative design, used as a logo or any other source to identify a personal possession. 
they use it sign their work so that people know who painted the piece

Just pretend that your talking to a empty room

Opposition of two melodic ideas is the  right answer