Say 'da-DUM da-DUM da-DUM' and you've got the rhythm of a heartbeat — and of iambic pentameter, five of those beats per line. Shakespeare's sonnets ride that steady pulse, which is part of why they sound so natural spoken aloud.
Fourteen lines, one argument
A Shakespearean sonnet has 14 lines in iambic pentameter, rhymed ABAB CDCD EFEF GG — three quatrains and a closing couplet. Iambic pentameter is five iambs per line, an iamb being an unstressed syllable followed by a stressed one (da-DUM), giving roughly ten syllables that echo natural English speech.
Sonnets usually 'turn' — a shift in argument or feeling. In Shakespeare's form the turn often comes at the final couplet, which resolves or reframes what the quatrains set up.
'Shall I / com-PARE / thee TO / a SUM / mer's DAY?' — five iambs; the stresses fall on every second syllable.
- Take the line 'But SOFT, what LIGHT through YON-der WIN-dow BREAKS'.
- Mark the stressed syllables.
- Count the stressed beats to name the meter.
What you should see: Five stressed beats in an unstressed-stressed pattern make it iambic pentameter — Shakespeare's default line.