In Macbeth, the witches speak in rhymed couplets most of the time:
Other times, they speak in unrhymed iambic tetrameter.
This is an inversion of how Macbeth and Lady Macbeth speak: they speak in blank verse most of the time, and in prose and rhymed verse rarely. Not only that, but Macbeth gets the best lines, then Lady Macbeth, then the witches.
The differences in verse meter shows the divisions in rank and status according to the Great Chain of Being. Here's the hierarchy of language:
1. Poetry (Blank Verse):
•Macbeth: thoughtful, poetic iambic pentameter (elevates him above rest)
•Lady Macbeth: plain, unimaginative iambic pentameter
2. Poetry (Rhyming Couplets)
•Witches: short, choppy iambic tetrameter
•Porter (servant): dark, bawdy common language; paragraphs
So, even though the witches are at the bottom of the social hierarchy, they outrank only the drunken Porter in terms of language and importance in the play. Their shortened lines show that they speak in a different tongue than nobility. Their rhymes show that their riddles are meant to be remembered, as they resonate and echo throughout the play, both in Macbeth's and the audience's memories.
The witches in Macbeth are unlike any of the other characters. They are "the weird sisters" - they have beards, and conjure up bizarre potions. Also, unlike the other characters, they are the only ones that use rhymed speech in the play. In a way, it sets them apart even more. It makes them seem slightly ridiculous, almost caricatures of the supernatural - jokes, in a way. They speak in rhyming couplets throughout (“Double, double, toil and trouble, / Fire burn and cauldron bubble” ), which also separates them from the other characters who mostly use blank verse to speak. A common technique that Shakespeare uses is to switch rhyme and meter to capture peoples' attention. Remember that these plays were viewed, not read, so playwrights had to use different techniques back in the day to make the audience pay attention. There were no special effects. What they did have were things like "iambic pentameter". Many times when Shakespeare wanted to make sure the audience was paying attention to an important part, he would go from blank verse to "iambic pentameter" which is very rhythmic in English.
So, he does this with the witches. He wants to call peoples' attention to what the witches say because the entire play hinges on their prophesy. If Macbeth had not heard their predictions, he would have gone his merry way and lived happily ever after..........well............maybe not.
The witches’ words seem humorous, like twised nursery rhymes. Despite the absurdity of their recipes (newt's eye, frog's toe, etc.), they are dangerous characters in the play. They are powerful and wicked and seem to have an uncanny sense of how to mess with Macbeth's head.