The Four Corner System has been used for many years in China  and  Japan  for 
classifying  kanji.  In  China  it  is  losing popularity in favour of Pinyin 
ordering.  Some Japanese dictionaries,  such as the  Morohashi  Daikanwajiten 
have a Four Corner Index. 

The following overview of the system has been condensed from the article "The 
Four Corner System:  an introduction with exercises" by  Dr  Urs  App,  which 
appeared in the Electronic Bodhidharma No 2,  February 1992, published by the 
International Research Institute for Zen Buddhism,  Hanazono  College.  (More 
examples will be added from that article in due course.) 

1. Stroke shapes are divided into ten classes:

                0       LID               е
                1       HORIZONTAL LINE   
                2       VERTICAL LINE     
                3       DOT               Ц
                4       CROSS             
                5       SKEWER            
                6       BOX               
                7       ANGLE             
                8       HACHI             Ȭ
                9       CHIISAI           

2. The Four Digits are derived from the Four Corners in a Z-shaped order. 

                A    B                   7  1     7  7
                         for example:            
                C    D                   2  9     2  2

Some examples:  2421   2122   7121   2733   0762  Ʊ 7722   4292

3.  A shape is only used once.  If it fills several corners, it is counted as 
zero in subsequent corners. 

Some examples:  6000   8060  ʬ 8022   2003   2690   6066   0096
4.  When the upper or lower half of a character consists of only one  (single 
or composite) shape,  it is,  regardless of its position,  counted as a left 
corner. The right corner is counted as zero. 

Some examples: Ω 0010  ͳ 5060   1017   0022   0024   2090   2050

5.  When there is no additional element to the four sides of  the  characters 
,  ,   (and sometimes ), whatever is inside these characters is taken 
for the lower two corners. 

Some examples:  7760   6080  Ԣ 6015   6010   7744   1060   2110

6.  The analysis is based on the block-style handwritten kaisho (ܴ)  shape 
of characters. 

(This needs attention, as  is 3027, not 1027.  The top stroke is treated as 
a Ц.) 

7. Some points to note when analyzing shapes: 

o Shape 0:  

When the horizontal line below a DOT shape (number 3) is connected to another 
stroke at its right-hand end (as in  ,  etc.) it is not counted as a  LID 
(number 0) but as a DOT. 

Examples:  3040   3520   3222

o Shape 6:

Characters  such  as  and  where one of the strokes of the square extends 
beyond it,  are not considered to be square (number 6)  shapes,  but  corners 
(number 7). 

Examples:  7710   3222   7710   8377  3010

o Shape 7:

Only the cornered end of corner shapes (number 7) is counted as 7. 

Examples:  7171   7222  2762  ȿ 7124

o Shape 8:

Strokes that cross other strokes are not counted as shape number 8 (Ȭ). 

Examples:  8043   7743   4003   8043   2143   9043

o Shape 9:

Shapes resembling shape 9, but featuring two strokes in the middle (as in the 
top part of  or ) or two strokes on one side (as in  or the bottom part 
of ) are not considered as 9 shapes. 

Examples:  4433   3290   3214

8. Some points to note when choosing corners. 

- when a corner is occupied by more than one independent or parallel strokes, 
the one that extend furthest to the left or right is  taken  as  the  corner, 
regardless of how high or low it is. 

examples:  1111   2124   0013   0022   3421   4721

- if there is another shape above (or, at the bottom of the character, below) 
the  leftmost  or  rightmost  stroke  of  a  character,  that  shape is given 
preference and is taken as the corner. 

examples:  3090   4040   6020   4040   3521  ¶ 4480

- when two composite stroke shapes are interwoven and each could be  regarded 
as a corner,  the shape that is higher is taken as the upper corner,  and the 
lower stroke as lower corner. 

- when a stroke that slopes downwards to the left or right  is  supported  by 
another stroke, the latter is taken as the corner. 

examples:  2740  ΢ 0073   1962   4464   4410   3424

- a left slanting stroke on the upper left is taken for the left corner only; 
for the right corner one takes a stroke more to the right. 

examples:  2740  ̶ 2350   6752  Ū 2762   2762   2772

9. Shape variations: (Dr App includes several pages of examples) 

10. The fifth corner:

In  order to differentiate between the several characters with the same code, 
an optional "fifth corner" is sometimes used. This is, loosely, a shape above 
the fourth corner which has not been used in any other shape.