A rhyme scheme is the pattern of rhymes that appear in a poem or piece of poetry. Rhyme schemes are written using letters or numbers to denote lines that rhyme with each other; for example, the following passage from Shakespeare's The Tempest has the rhyme scheme AAAAABB (or 1111122):
Where the bee sucks, there suck I;
In a cowslip's bell I lie;
There I couch when owls do cry.
On the bat's back I do fly
After summer merrily.
Merrily, merrily shall I live now
Under the blossom that hangs on the bough.
Note that the fifth line (ending 'merrily') is an eye-rhyme. If one chose not to count this as a rhyme, the rhyme scheme would be written AAAABCC.
Some kinds of poem are defined by their rhyme scheme, for example:
Half-rhymes (eg eat/beak or cat/slap) can be differentiated from full rhymes by using lower-case letters. For example, in the following extract from 'Up the Junction' by Squeeze, the rhyme scheme could be written aabbCCdd:
I never thought it would happen
With me and the girl from Clapham
Out on the windy common
That night I ain't forgotten
As she dealt out the rations
With some or other passions
I said 'You are a lady'
'Perhaps,' she said, 'I may be...'
Note that the fifth and sixth lines rhyme fully, so are denoted by uppercase letters, whereas the rest are half-rhymes and so are represented using lowercase.