About the four-corner system

The four-corner system is a system for encoding kanji into numbers using the shapes of the four corners of the character. The corners are taken from top to bottom and left to right. The four-corner code for 口 is 6000, where the 6 means "square shape at top left", and the three 0s indicate that the square shape was previously used. The four-corner code for 品 is 6066, indicating a square at the top left, with the same square also constituting the right corner, followed by two more squares indicated by 66 at the bottom left and right.

A horizontal line is indicated by a 1, and a corner by 7, so 1771 is the four-corner code for 乙, which has corners at the top right and bottom left, and horizontal lines at the other two corners. A vertical line is indicated by a 2, so 川 is 2200, where the twos indicate vertical lines and the 0s indicate that the vertical lines were previously used at the top.

A line which crosses another line is indicated by 4, so 力 is 4002 indicating the crossed line, used again twice, and then by 2 for the vertical line at the bottom right. A line which crosses two other lines is indicated by 5, so 丈 is 5000 for the downward line crossing two others.

• Dylan Sung's introduction
• Jim Breen's Kanjidic documentation