Arrays versus nested lists

The Split (↓) primitive turns an array into an array of lists, reducing its rank by one and introducing an extra level of nesting.

      2 3 5⍴⎕A  ⍝ Rank 3 array
ABCDE
FGHIJ                         ↓2 3 5⍴⎕A  ⍝ Rank 2 array of lists
KLMNO                   ┌─────┬─────┬─────┐
                        │ABCDE│FGHIJ│KLMNO│
PQRST                   ├─────┼─────┼─────┤
UVWXY                   │PQRST│UVWXY│ZABCD│
ZABCD                   └─────┴─────┴─────┘

      ↓↓2 3 5⍴⎕A  ⍝ List of lists of lists
┌───────────────────┬───────────────────┐
│┌─────┬─────┬─────┐│┌─────┬─────┬─────┐│
││ABCDE│FGHIJ│KLMNO│││PQRST│UVWXY│ZABCD││
│└─────┴─────┴─────┘│└─────┴─────┴─────┘│
└───────────────────┴───────────────────┘