Regardless of where a definition is stored, a custom hatch pattern has the same structure.
It has a header line with a name, which begins with an asterisk and is no more than
31 characters long, and an optional description.

*pattern-name, description

a pattern definition also has one or more line descriptors of the following format:

angle, x-origin,y-origin, delta-x,delta-y,dash-1,dash-2, …

The standard hatch pattern ANSI31 looks like:

and is defined as follows:

*ANSI31, ANSI Iron, Brick, Stone masonry
45, 0,0, 0,.125

The pattern name on the first line, *ANSI31, is followed by a description: ANSI Iron, Brick, Stone masonry. This simple pattern definition specifies a line drawn at an angle of 45 degrees,
the first line of the family of hatch lines is to pass through the drawing origin
(0,0), and that the spacing between hatch lines of the family is to be 0.125 drawing
units.

Hatch pattern definitions follow these rules:

• Each line in a pattern definition can contain up to 80 characters. You can include
  letters, numbers, and the special characters underline (_), hyphen (-), and dollar
  sign ($). However, you must begin a pattern definition with a letter or number, not
  a special character.
• Blank lines and text to the right of a semicolon are ignored.
• Each pattern line is considered to be the first member of a line family, created by
  applying the delta offsets in both directions to generate an infinite family of parallel
  lines.
• The delta-x value indicates the displacement between members of the family in the direction of
  the line. It is used only for dashed lines.
• The delta-y value indicates the spacing between members of the family; that is, it is measured
  perpendicular to the lines.
• A line is considered to be of infinite length. A dash pattern is superimposed on the
  line.

The process of hatching consists of expanding each line in the pattern definition
to its infinite family of parallel lines. All selected objects are checked for intersections
with any of these lines; any intersections cause the hatch lines to be turned on and
off as governed by the hatching style. Each family of hatch lines is generated parallel
to an initial line with an absolute origin to guarantee proper alignment.

If you create a very dense hatch pattern, the program may reject the hatch and display
a message indicating that the hatch scale is too small or its dash length too short.
Change the maximum number of hatch lines by setting the MaxHatch environment variable
to a whole number between 00 and 10000000 (ten million):

• In AutoCAD LT, use SETENV and enter a valid number.
• In non-LT products, use the AutoLISP expression (setenv “MaxHatch” “n“) where n is a valid number.