Most ballads use one of three different types of rhyme: abac, aabb, or abcb.
The first type of rhyme, abac, is found in ballads that include a chorus in the verse: the first and third lines of each verse rhyme, while the second and fourth lines, the chorus, are the same in every verse.
She went down below the thorn
Fine Flowers in the Valley
And there has she her sweet babe born
And the green leaves they grow rarely
the second type, aabb, the first and second lines rhyme with eachother, as do the third and the fourth lines. For example:
As I was walking al alane
I saw twa corbies makin' mane
the tane untae the tither did say
where shall we gang and dine the day?
It's possible that this type of ballad evolved from the first
The third type of rhyme, abcb, is the most common type of rhyme found in Child's ballads. In this rhyme scheme, only the second and fourth lines rhyme:
Her breath was strang, her hair was lang
And twisted twice about the tree
And with a swing she came about
Come to Craigy's sea and kiss with me