Say "to BE or NOT to BE, that IS the QUES-tion" out loud and the rhythm is almost impossible to miss: five pairs of syllables, each pair a soft beat followed by a hard one — da-DUM, da-DUM, da-DUM, da-DUM, da-DUM. That pattern is iambic pentameter: an iamb is one unstressed syllable followed by one stressed syllable, and "penta" means the line repeats that unit five times, for ten syllables total. It's the metre behind the overwhelming majority of Shakespeare's dialogue, and behind an enormous share of English poetry more broadly, and it earns that dominance for a reason that has nothing to do with being ornate — it's close to how English already sounds when people just talk.
Verse that hides inside ordinary sentences
Say a huge number of common English words and phrases out loud and they naturally fall into that same soft-hard shape: "toDAY," "beCAUSE," "the DOG ran HOME." English is a stress-timed language, meaning some syllables in a sentence get more emphasis than others as a basic feature of how the language is spoken, not as a performance choice. Iambic pentameter doesn't fight that underlying rhythm — it organises it into a regular, countable pattern of five beats per line, close enough to natural speech stress that a well-written line can be spoken as if it were simply talking, while still quietly keeping a beat underneath. That's the balancing act the metre is built to pull off: regular enough to register as verse, loose enough to still sound like a person, not a chant.
A tool for signalling who's speaking, and how
Shakespeare didn't write everything in iambic pentameter, and the choice of when not to is itself doing work. Prose shows up for lower-status characters, comic scenes, and moments meant to feel unpolished or plain-spoken; rhymed verse marks especially heightened or ceremonial moments; unrhymed iambic pentameter — blank verse — carries the bulk of the serious dialogue for the play's central characters. A character shifting from verse into prose partway through a scene is a signal in itself, visible on the page even before an audience hears the change in delivery, and Shakespeare used that shift deliberately rather than sticking to one register throughout a play.
What we're still unsure about
The "sounds like a heartbeat" comparison is a popular and genuinely useful way to teach the rhythm — da-DUM does resemble a pulse — but it's a teaching device, not a historical account of why the metre exists. Iambic pentameter's actual origins trace to earlier English alliterative and stress-based verse traditions, adapted alongside influence from syllable-counting conventions in French and Italian poetry, long before anyone framed it in terms of cardiac rhythm. Treating the heartbeat comparison as the reason poets chose this metre, rather than as a modern mnemonic for remembering how it sounds, would be mixing up a helpful analogy with an origin story it was never meant to tell.
This sits inside Metre, Rhythm & Prosody, one of seven topics in Poetry, one of five domains in Literature, one of seventeen subjects the app can quiz you on.