IBM 5110 User Manual page 24

The rank of an array is the number of coordinates it has, or the
number of indices required to locate any element within that array.
Scalars are rank O. Vectors have a rank of 1, matrices have a rank of
2, and N-rank arrays have a rank from 3 to 63 (where N is equal to the
rank). N-rank arrays, like matrices, are generated by providing as the
left argument a number indicating the length for each coordinate (for
example, planes, rows, and columns). The following examples show
how to generate 3-rank arrays. Note that the elements taken from the
right argument are arranged in row order:
ABeD
EFGH
I ,..II{ I...
MNDP
(~I~BT
UVl,JX
AB
CD
EF
GH
I.J
1{1...
MN
DP
QI~
ST
UV
WX
A~·ABCDEFGHIJKl...MNOPQRSTUVWXYZ·
2 :3
I.J.(.>(.:}
\'-----
2-Plane, 3-Row, 4-Column Array
1·1·
:.~ ~.~ (.>
A
\'-----
4-Plane, 3-Row, 2-Column Array
APL Language Elements 19