Watch the Open and Closed online guitar lesson by Matthieu Brandt from Songwriting on Guitar
The structure of the melodic phrases has an influence on the open- and closed feel of a song section. Playing with these patterns can give your song the general feel it needs. Symmetrical strucures (like abab) will often create a more closed feel than asymmetrical structures (abb). In effect you are guiding the expectation of the listener with these structures. The melodic structure does not have to follow the underlying harmonic structure. In modern pop- and rock songs, where attention is a commodoty - assymmetric structures occur a lot, to speed the song up. This can be done by removing one or two 'empty' ending bars of a section or deviding a symmetric section in two or three phrases with different lengths.