Couplets can be of any length, And shorter size gives greater strength Sometimes – but sometimes, willy-nilly, Four-beat couplets sound quite silly. (Some lines really should stay single: Feminine rhymes can make them jingle.)
If you read it, choose the third or fourth editions, which include an added section of examples taken from centuries of poetry that exhibit the patterns he has described.
Worth Reading
Rhyme’s Reason: A Guide to English Verse by John Hollander
This is a book my mother gave me when I first began writing poetry in college. I remember having a high opinion of it then.
A brief but comprehensive survey of the forms, rhyme schemes, and metric patterns of English verse. What distinguishes Rhyme’s Reason from other such books is Hollander’s brilliant illustrations of each variation with a definition that is, itself, an example of the form.
On couplets, for example: