Approximate rhyme, sometimes called near rhyme, occurs between two words that are similar in sound, but do not rhyme exactly. It contrasts with perfect or true rhyme, in which the words do, in fact, rhyme. The device is used in poetry, rhymed verse and songwriting when a true rhyme is inappropriate to the meaning of the piece or when no true rhyme exists.
As with true rhyme, approximate rhyme usually occurs in a final, stressed syllable. It is generally mono-syllabic, meaning that the near rhyme applies only to a single syllable. Multisyllabic near rhyme, which is approximate rhyme in which more than one syllable of two words almost rhyme, is quite rare.
An example of true rhyme would be the words "true" and "blue." Examples of near rhyme would be "breadth" and "depth." While these two words do not actually rhyme, the similar sound approximates rhyme.
One major reason to use approximate rhyme is when there is simply no word that rhymes perfectly with the word in question. When this happens, near rhyme allows the writer to stick to his original word choice by stretching the rules slightly. Examples of English words that have no available perfect rhyme include angst, bulb, filmed, oblige and wolves. Several number-place indicators also lack a perfect rhyme in the English language. These include fifth, sixth, eighth and twelfth.
Another purpose of approximate rhyme is to allow the use of a specific word, even though it does not quite fit into the rhyme scheme. This most often occurs when the writer wants to convey a certain image or emotion and needs a specific word or type of word to do so. In some cases, there may be a true rhyme available, but it might not fit with the mood or flow of the poem, or it might not have a meaning that fits with the story or message.
Generally, near or approximate rhyme can be used in place of perfect rhyme in any rhyme scheme. It can be used throughout a piece or can be intermingled with lines that rhyme perfectly. Certain formal poetry forms do not allow the use of near rhyme, instead requiring that all rhymes be perfect. This does not mean that a writer cannot use near rhyme in these formats, it merely means that, by doing so, he creates a piece that does not conform to the standards of the form.