The form consists of three quatrains and a couplet.
The couplet generally introduced an unexpected sharp
thematic or imagistic “turn”.
The usual rhyme scheme was a-b-a-b, c-d-c-d, e-f-e-f, g-g.

This example, Shakespeare's Sonnet 116, illustrates the form:

Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove.
O no, it is an ever fixed mark
That looks on tempests and is never shaken;
It is the star to every wand'ring barque,
Whose worth's unknown although his height be taken.
Love's not time's fool, though rosy lips and cheeks
Within his bending sickle's compass come;
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
I never writ, nor no man ever loved.

A variant on this form is the Spenserian sonnet,
named after Edmund Spenser (c.1552–1599)
in which the rhyme scheme is a-b-a-b, b-c-b-c, c-d-c-d, e-e.

This example is taken from Amoretti
Happy ye leaves! whenas those lily hand
Happy ye leaves! whenas those lily hands,
Which hold my life in their dead doing might,
Shall handle you, and hold in love's soft bands,
Like captives trembling at the victor's sight.
And happy lines! on which, with starry light,
Those lamping eyes will deign sometimes to look,
And read the sorrows of my dying sprite,
Written with tears in heart's close bleeding book.
And happy rhymes! bathed in the sacred brook
Of Helicon, whence she derived is,
When ye behold that angel's blessed look,
My soul's long lacked food, my heaven's bliss.
Leaves, lines, and rhymes seek her to please alone,
Whom if ye please, I care for other none.