What do you call a measure that is incomplete in beats and found at the beginning of a song?

The Anacrusis

Explanation:

The Anacrusis (incomplete measure at the beginning of a melody) does not have a Measure Number. It is part of the final measure (the incomplete measure at the end of the melody). Conducting is an important "tool" to help students feel the Pulse and the Beat.

lines and shapes are a part of the visual elements which can be played with and used to create a mood or feeling in a composition.

some of the other elements include the pattern, color, tone and texture. all these elements serve together as the construction blocks which are used by the artists to create artwork.

lines are used to form drawings and different shapes are formed when these lines are used in certain lengths or angles. so the artist can play with these elements of art to create and express different moods, thus conveying different  meanings.