The answer is: They all contain three quatrains and a couplet.
Shakespeare´s sonnets, except for sonnets 99, 126, and 145, all have the same structure: three quatrains and a final couplet.
A quatrain is a four line stanza. A couplet is a pair of two successive lines that usually rhyme.
Shakespeare usually develops and idea or figure in each of the quatrains (presenting a turn at the beginning of the third quatrain), and the couplet, finally, tends to shift these ideas to shed new light on the hermeneutic process.
