Since there are several Lisp data types that can be denoted by a pointer, there must be some way to distinguish them. There are several ways to do this.

1. Each datum can contain a type field or tag that tells the type of the datum.

2. Each pointer can contain a type field or tag that tells the type of the datum pointed to.

3. Each pointer can implicitly contain the type by keeping only one type of data on a given page of computer memory; the type is then found by table lookup using the high-order bits of the pointer address.

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