Final answer:
The rhyme scheme of lines 1-8 of Shakespeare's Sonnet 18 is ABABCDCD, characteristic of a Shakespearian sonnet's octave.
Step-by-step explanation:
The rhyme scheme of lines 1-8 of Shakespeare's Sonnet 18 follows the pattern ABABCDCD. This structure is characteristic of the Shakespearian sonnet, which is traditionally composed of 14 lines in iambic pentameter. The first 8 lines, known as the octave, typically introduce a problem or theme which is then followed by a resolution or counter-theme in the final six lines, the sestet. The transition between these two sections is marked by a 'volta' or turn in the argument or mood.