A Haiku contest rule says that the lines that could be used are any of the following rhyme schemes: a-a-a, a-b-a, a-a-b, a-b-b, or a-b-c. What does this mean?
The rules mean that you can write a haiku with a variety of different ways to rhyme the words at the end of each line.
Rhyme scheme in poetry means which words (usually at the end of a line) rhyme with each other. The rhyming words are given one letter (for example, A) that match their rhymes.
A haiku is a style of poem that has 5 syllables in the first line, 7 in the second line, and 5 again in the last.
On a windy day
A gust lifted up my hat
and stole it away
This haiku has the rhyme scheme A-B-A because the words at the ends of the 1st and 3rd line (day and away) rhyme, while the word at the end of the 2nd (hat) does not.
An A-A-A rhyme scheme haiku would use rhyming words at the end of all three lines (blue, flew, new - for an example).
A haiku with the rhyme scheme A-B-C would have no rhyming words at the end of lines at all.