This is a common rhyme scheme used in many poems, including ballads. It's a simple and effective scheme that helps create a sense of rhythm and flow.
Here's an example from the poem:
* A "She sang a song, the Nightingale,
* B "A simple song, a little thing"
* A "But it was sweet, the Nightingale's song,
* B "And full of joy, and full of spring."
The rhyme continues through the rest of the poem, creating a pleasing and predictable rhythm.