The sonnet exemplifies Shakespeare's innovation of the English form of the genre. Although written in the rhyme scheme abab cdcd efef gg which is typical of the English form, sonnet 18 has a strong pause (or volta) between the first eight lines (the octave) and the last six (the sestet). This is typical of the Italian or Petrarchan sonnet. The pause in the text is signalled by the colon and by the conjunction BUT which introduces a contrast with what has been said earlier. The pause thus corresponds to the introduction of new imagery or a new topic. Other innovative sonnets are 99 which has fifteen rather than fourteen lines and 126 (six couplets).
Sonnet 18 also introduces a variation in terms of content compared to the first seventeen poems. The speaker expresses for the first time his ambition to make youth immortal in his lines, thus adopting a new perspective.