Couplets are any two lines working as a unit, whether they comprise a single stanza or are part of a larger stanza. Most couplets rhyme (aa), but they do not have to. There are several set forms of the couplet and a myriad of variations based on line length and meter. All of the following rhyme "aa":
How To You can do almost anything with a couplet. They can stand as single thoughts, meaning they can exist on their own, outside of the poem, or they can be enjambed, relying on the previous and succeeding couplets to be complete. Most open form couplets are written this way, and a rhyme scheme should play no bearing on how couplets are or are not interlocked.
The couplet can be a very lonely stanza, minimalistic. Poems whose content is melancholy or depressing, for example, can make good use of the couplet because--on the page--there is a lot of white space, emptiness, as opposed to writing in quatrains where the stanzas are blocks which limit the white space. As well, because the couplet can be so small, it is a good idea to pack it full of image and emotion, like a hard punch packed in a tight space, very concentrated. If the power in a couplet is not contained to the couplet, then you have a quatrain or something larger. This doesn't mean the idea and emotion cannot flow between or through couplets, I am only suggesting that each couplet be a powerful, emotionally-intensive unit to the whole.
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