For all his inventiveness and rule-breaking and bar-setting and anarchy in his staged works, Shakespeare stuck faithfully (and almost rigidly) to the iambic pentameter poetical form of his time (AB AB CD CD EF EF GG rhyming pattern) using precisely seven rhyming sounds and exactly 140 syllables per poem. Within that, he tried to be fresh and innovative but he remained faithful to the conventions of the poetical period.
Taken from The Great Courses’ Shakespeare: The Word and the Action