To understand Shakespeare's innovations to sonnets, it is necessary to understand the origin of sonnets. The original sonnet for was Petrarchan, meaning it was the form Petrarch used when he first developed the sonnet. He developed the sonnet as having fourteen lines and two parts, or stanzas, to it.
The first stanza was developed as an octave (eight lines). It had a set rhyme scheme that Petrarch did not vary. That Petrarchan rhyme scheme is abbaabba. The second stanza was developed as a sestet (six lines) with a rhyme scheme that may vary between five or six possible schemes, such as cdecde or cdeced or cdcdcd or cddcdc or cdcedc. Petrarch used no couplets.
Petrarch followed the poetic convention that a change in rhyme scheme accompanies a change in subject matter, therefore, the octave and sestet cover different subject of the topic undertaken in the poem. This change in subject matter is called the volta, or turn, and in Petrarchan sonnets is at the ninth line.
Shakespeare was well aware of the Petrarchan sonnet form because it was the form that entered England from the writings of Petrarch and other Italian sonnet poets. Shakespeare innovated changes to the form, thereby creating a purely Shakespearean sonnet form, also called an English sonnet form, that he consistently composed sonnets in and that other English poets of the Renaissance borrowed for their own sonnets.
Shakespeare changed the structure to three quatrains (total 12 lines) and an ending couplet. He consequently changed the rhyme scheme to abab cdcd efef gg, remembering that the poetic convention is that there is a change of subject at each rhyme change. Since Shakespeare changed the form and rhyme scheme, the subject matter has more flexibility and is therefore less dictated by the sonnet form.
For instance, in Petrarchan sonnets, the subject matter changes at line 9 and the sestet provides the solution to the problem presented in the octave: subject and form are closely tied. In the Shakespearean sonnet form, there may be two examples of the subject matter presented, as in Sonnet 29, with a turn to the solution in the third quatrain. The couplet then, provides the ultimate solution, the lesson learned so to speak, such as when Shakespeare writes in "Sonnet 29": "For thy sweet love remembered such wealth brings / That then I scorn to change my state with kings." As a result of structure changes, the Shakespearean sonnet subject matter is less tied to the innovated English sonnet form.