What is the rhyme scheme in Shakespearian Sonnet 20?

Shakespeare's preferred sonnet structure was so unique that it has a name of its own: the Shakespearean sonnet! In fact, Shakespeare did not invent the Shakespearean sonnet. It preceded him and is also widely known as the English or Elizabethan sonnet. The name now pays homage to Shakespeare because he greatly helped in the popularization of its particular form.
Shakespearean (or English) sonnets share attributes of other sonnets, including the poetic meter known as iambic pentameter. Iambic pentameter refers to lines that are ten syllables each (or "decasyllabic"). Each line consists of five "feet," or groupings of two syllables. Each foot consists of one unstressed or "soft" syllable, followed by a stressed or "hard" syllable. Explanatory foot notation often writes out a foot as the sound-expression "da DUM."
What is distinctly "Shakespearean" about a Shakespearean sonnet is not its use of iambic pentameter but its rhyme scheme. Shakespearean sonnets consist of four stanzas. In each of the first three stanzas, there are four lines, and every other line rhymes. The final stanza consists of only two lines, and they rhyme with each other. In rhyme notation, this feature is written out as "ABAB CDCD EFEF GG."
The highly flexible and playful structure of the Shakespearean sonnet lends itself to a broad range of themes and images. Scholars of poetry argue that Shakespeare used the form to experiment with sexual meaning. Indeed, sexual connotations are abundant in Sonnet 20. In Shakespeare's contemporary world, there were no terms like "gay" or "bisexual" to describe sexual orientation. Gender identity also had a limited explicit vocabulary. The form of the sonnet facilitates the making of new associations between rhyming or parallel language and forces the use of atypical combinations of words. The form also accommodates ambiguity. It was taboo to have sexually "deviant" behavior then, just as it is in some cultures now. The form thus allowed Shakespeare to both conceal and illuminate his meaning, depending on the attitudes and preconceptions of those who read him.