Note to Hamlet, 2.2.428-429: "a piece of uncurrent gold, be not cracked within the ring"

a piece of uncurrent gold: a gold coin that is not lawful currency.  cracked within the ring: Coins were typically stamped with a sovereign's head inside a circle; if a coin had a crack from the edge into the circle (was "cracked within the ring") it was "uncurrent," not lawful currency. Hamlet is punning on the word "cracked" because when a boy's voice cracks—changes from boyish treble to adult baritone—that boy can no longer play women's parts.