The notation [m:n] is used in this manual to describe fields within data words. The 48 accessible bits of a data word are considered to be numbered, with the leftmost bit numbered as bit 47 and the rightmost bit numbered as bit 0. In the notation [m:n], m denotes the number of the leftmost bit of the field being described, and n denotes the number of bits in the field. For example, the field indicated by the shaded area in Field Notation, [29:6] (bits 29 through 24) is described as [29:6].

All data words have a field associated with them that is called the tag of the word. The tag identifies the type of the data word; that is, whether the word is an operand, descriptor, and so on. The tag is not accessible to ALGOL programs.

Hexadecimal format is used extensively in this manual to indicate word contents. This format is particularly suited to describe the value of a data word, because each hexadecimal digit indicates the contents of a 4-bit field. Such fields can be visualized as the columns in Field Notation, [29:6].