What is the difference in the rhyme scheme of Shakespeare's Sonnet 18 from a Petrarchan rhyme scheme?

Shakespeare's sonnets are written according to the English
sonnet structure that was developed during the Renaissance by poets like Wyatt, the Earl
of Surrey, and Spenser. The originator of the sonnet form was Petrarch, an Italian poet
who lived 200 years before the English sonnet
poets.

Petrarch used a rhyme scheme that fitted his sonnet
structure of fourteen lines comprised of an octave (8 lines) followed by a sestet (6
lines), with no rhyming couplet. The Petrarchan rhyme scheme was
abbaabba in the octave, followed by one of several options
for the sestet. Some of those options
were:
cddcdc
cdeced
cdecde
cdcdcd
cdcedc

The
English sonnet structure of fourteen lines was modified to fit the English language and
taste. It is structured as three quatrains (4 lines) and an ending rhyming couplet (2
lines).

The English sonnet rhyme scheme fits this new
structure. Shakespeare's Sonnet 18 has an English sonnet rhyme scheme that is different
from the Petrarchan rhyme scheme in that each quatrain is two sets of alternating
rhyming lines, which produces a rhyme scheme of abab cdcd efef
followed by the rhyming couplet, which is
gg.