datatype preceded by underscore creates array

Is there any special meaning to preceding a datatype (or at least some
datatypes) in a table or function definition by underscore that is a
synonym for an array? I can't see it documented anywhere. Below are some
examples. The other question is why "_int4" parses to int[], but "_int"
does not, etc. This is on PostgreSQL 8.1.3 Linux.

gp=> create table a1 (b _int4);
CREATE TABLE
gp=> \d a1
Table "mnp.a1"
Column | Type | Modifiers
--------+-----------+-----------
b | integer[] |

gp=> create table a2 (b _char);
CREATE TABLE
gp=> \d a2
Table "mnp.a2"
Column | Type | Modifiers
--------+----------+-----------
b | "char"[] |

gp=> create table a3 (b _int);
ERROR: type "_int" does not exist

gp=> create table a4 (b _char(1));
ERROR: syntax error at or near "(" at character 25
LINE 1: create table a4 (b _char(1));
^