What is an ABCB rhyme scheme?

The letters stand for the ends of the lines. This pattern describes a four-line verse with ending rhymes as follows:

A

B

C

B

This means that the second and fourth lines rhyme but the other two don't. This nursery rhyme is an example of an ABCB scheme:

Mary had a little lamb,

Its fleece was white as snow,

And everywhere that Mary went

The lamb was sure to go.

A - lamb

B - snow

C - went

B - go

ABCB Rhyme Scheme is also called "Simple 4-Line"