Note to Hamlet, 5.1.137: "How absolute the knave is! we must speak by the card"
The gravedigger's wit here consists of insisting that there is absolutely only one correct way to speak of what is going on. Thus all of Hamlet's questions about who is to be buried in the grave are deemed wrong by the gravedigger because it's not a "who," but a "what" the remains of a person which are to be buried.
Hamlet concludes that the gravedigger must be answered with the same sort of absolute precision, "by the card." The "card" which Hamlet has in mind is a compass card, such as the one shown below. The needle of the compass is mounted in the center of the card and indicates the precise direction of travel.
 | Compass Card |
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