E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Ophelia says to the queen, You may wear your rue with a difference. In heraldry differences or marks of cadency indicate the various branches of a family.
(1) The eldest son, during the lifetime of his father, bears a label (or lambel), i.e. a piece of silk, stuff, or linen, with three pendants, broader at the bottom than at the top.
(2) The second son bears a crescent.
(3) The third, a mullet (or star with five points).
(4) The fourth, a martlet.
(5) The fifth, an annulet.
(6) The sixth, a fleur-de-lis.
(7) The seventh, a rose.
(8) The eighth, a cross-moline.
(9) The ninth, a double quatre foil.
Ophelia says both she and the Queen are to wear rue: the one as the affianced of Hamlet, eldest son of the late king; the other as the wife of Claudius his brother, and the cadet branch. The latter was to
have a difference, to signify it was a cadet branch. I [says Ophelia] shall wear the rue, but you [the Queen] must now wear it with a difference.