Jane Shaw Whitfield in Whitfield's University Rhyming Dictionary, Barnes and Noble Books, ©1951

Rhyme is "the correspondence, in two or more words or verses, of terminal sounds beginning with an accented vowel, which, in modern English usage, must be preceded by different consonant sounds, or by a consonant in one case and none in the other." Rhyme is the chiming of identical terminal sounds of words. It may occur regularly at the ends of lines, and within lines as well [internal rhyme], as in many of the poems of Edgar Allan Poe. In English, rhyme is based on an accented vowel sound. This rhyme sound usually consists of accented vowel plus final consonant, and in that case is called a masculine, or one-syllable, rhyme. It may be composed of accented vowel (with or without consonant) followed by one or two unaccented syllables. A two-syllable rhyme is called a feminine rhyme, while a three-syllable rhyme has no special name.

In rhyming, there are only two rules:

Thus, vacation and oblation are acceptable rhymes, while vacation and extrication are not, being mere repetitions of -cation, instead of rhymes on -ation.

In modern poetry, very frequently a kind of offbeat rhyming is used to good effect; these so-called "imperfect" rhymes are considered to be words which contain the rhyme sound, but in which the sound is not accented.

Rhymes and Syllables

Single (masculine) or exact rhymes: primary or secondary accents falls on last syllable as in these pairs: gate-late; own-bone; aware-hair; applause-gauze; go-throw.

Double (feminine) rhymes: primary or secondary accent falls on next to last syllable; that is, the rhyme normally falls on an accented and unaccented syllable as in these pairs: going-throwing; merry-cherry; but they may fall on two accented syllables, as in "ping-pong" - "sing-song."

Triple rhymes: primary or secondary accent falls on second from last syllable; that is, the rhyme normally falls on an accented syllable and two following unaccented syllables, as in these pairs: radium-stadium; changeable-rearrangeable; flavoring-unwavering.

Robert Wallace in Writing Poems , Little, Brown and Company, ©1982:

By definition, "rhyme" is an identity in two or more words of vowel sound and of any following consonants (or syllables in the case of two- or three-syllable rhymes). Rhymes normally fall on accented syllables.

English is not as easy language to rhyme. (By contrast, the problem in Italian is to keep from rhyming.) There are a number of familiar words for which there are no natural rhymes, like "circle" or "month." For some words there is only one natural rhyme: "strength-length," "fountain-mountain." And a word as much used as "love" offers only meager possiblilites: "above," "dove," "glove," "shove," "of." Despite a poet's best contortions, it is hard to make such rhymes fresh; consequently, unrhymed verse is standard, as the much-used blank verse of Shakespeare's plays and many of Frost's dramatic monologues. Other properties of sound, like alliteration and assonance, can make unrhymed verse as musical or effective as need be.

The difficulty of rhyme in English has opened up a wide variety of inexact rhyme--often termed "off"- or "slant-rhymes"--[or "partial-rhymes"] that the poet may use with considerable freshness. One device is terminal alliteration, as in love-move, bone-gone, what-bat, or chill-full. [Rhymes such as love-move and bone-gone are also referred to as "eye rhymes" because it is the eye that sees the similarity in spelling instead of the ear that hears the similarity of sound.]

[Consonance: The repetition of similar consonant sounds in a group of words. The term is often used for a form a partial rhyme in which the consonants are the same but the vowels are different. Sometimes the term is limited to the repetition of final consonant sounds, also called terminal alliteration.]

Another is consonance (identity of consonants with different main vowels), as in bad-bed, full-fool, fine-faun, or summer-simmer; or near consonance as in firm-room, past-pressed, or shadow-meadow. There is assonance, of course, as in bean-sweet or how-cloud.

Rhyming accented with unaccented (or secondarily accented) syllables is also a frequent method of off-rhyme, as in see-pretty, though-fellow, full-eagle, fish-polish them-solemn, under-stir. There is no need to be systematic about the varieties of off-rhyme. Anything will do in the right context.