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So far you have dealt with atoms that are either numeric or character. The monad < (box) introduces a new type of atom called boxed. The monad < applies to any noun and returns an atom that is a box which contains the argument.

An array is either a numeric array that contains numbers, or a literal array that contains characters, or a boxed array that contains boxes. Arrays of numbers and characters are referred to as open to distinguish them from boxed arrays.
```   b =. < 2 3 4
\$b		NB. an atom has empty shape
```
A boxed array is displayed in a box.
```   b
+-----+
¦2 3 4¦
+-----+
c =. < 4 7 9
d =. b , c	NB. append
d
+-----+-----+
¦2 3 4¦4 7 9¦
+-----+-----+
\$d		NB. list with shape 2
2
(< 2 3 4) , < 4 7 9
+-----+-----+
¦2 3 4¦4 7 9¦
+-----+-----+```
Arrays of different types (numeric, character, and boxed) cannot be appended to one another.
```   'a' , 3
¦domain error
¦   'a'    ,3
3 , < 2 3 4
¦domain error
¦   3    ,<2 3 4```
Boxed arrays are of the same type and can be appended no matter what they contain.
```   (< 'abc') , < 4 5 6
+---+-----+
¦abc¦4 5 6¦
+---+-----+
(< 2 3 \$ 'abcdef') , (< i. 3 4) , < 23
+---+---------+--+
¦abc¦0 1  2  3¦23¦
¦def¦4 5  6  7¦  ¦
¦   ¦8 9 10 11¦  ¦
+---+---------+--+```

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