ABAB Song Form:
Traditionally it starts off with an A section composed of 8 bars followed by a B section of 8 bars as well. Then another A and B section follows.
Example: In "Fly Me to the Moon" by Frank Sinatra, you'll notice that the A section starts with the line "Fly me to the moon," the B section begins with "In other words, hold my hand," then another A section ("Fill my heart with song") and B section ("In other words, please be true"). The song was extended by repeating the second A and B section. Listen to a song sample courtesy of YouTube.
ABAC Song Form:
The classic structure of this song is similar to that of the ABAB form. It begins with an 8-bar A section followed by a B section that also has 8 bars. Then it returns to the A section before going into a C section. The first bars of the C section begins melodically similar to the B section before it changes.
More on ABAC:
This form is often used in stage musicals or moviesExample:
"Moon River" by Andy Williams. If you listen closely you'll notice that the C section starts off with a line melodically and lyrically similar to the B section ("Two drifters off to see the world). Then it changes melodically and lyrically ("We're after the same rainbow's end"). Listen to a song sample
courtesy of YouTube.
ABCD Song Form:
Refers to a type of song where the melody changes and the story progresses for each section.
Example: An example of this is Richard Rodgers and Oscar Hammerstein's "You'll Never Walk Alone" (listen to song sample). You'll notice that for each section the melody changes.