These computers had two instruction formats, one of which, the Type A, had a short, 3-bit, operation code prefix and two 15-bit fields separated by a 3-bit tag.
8 The 704 hardware had special instructions for accessing the address and decrement fields in a word.[2]: p.
26 As a result it was efficient to use those two fields to store within a single word the two pointers needed for a list.[3]: Intro.
[4][5] Precursors[6][7] to Lisp included functions: each of which took a machine address as an argument, loaded the corresponding word from memory, and extracted the appropriate bits.
The prefix and tag parts were dropped in the early stages of Lisp's design, leaving CAR, CDR, and a two-argument CONS.
In Lisp, however, the cons cell is not used only to build linked lists but also to build pair and nested pair structures, i.e. the cdr of a cons cell need not be a list.