Sinclair ZX81 User Manual page 75

Number whose name is longer than one letter:
Array of numbers:
The order of the elements is:
first, the elements for which the first subscript is 1
next, the elements for which the first subscript is 2
next, the elements for which the first subscript is 3
& so on for all possible values of the first subscript.
The elements with a given first subscript are ordered in the same way using the second subscript, & so
on down to the last.
As an example, the elements of the 3 x 6 array B in chapter 22 are stored in the order
B(1,1),B(1,2),B(1,3),B(1,4),B(1,5),B(1,6),B(2,1),B(2,2),...,B(2,6),B(3,1),B(3,2),...,B(3,6).
Control variable of a FOR-NEXT loop:
String: