Unlike the rule of the Cangjie system, physical or natural brush strokes can be broken here. The only relevant factor for the Four Corner System are the four "corners" of a character. Every corner is given one of ten numbers. The general rules for these numbers are:
The rules of the Four Corner System are the following:
||亠 head with dot on the top
"joker" for a stroke already used (see rules, down)
small hook from left to right (left bottom)
乚 bend hook to the right (right bottom)
丿亅 hook or stroke to the left
乀 swing to the right
||乂十 crossed line of every kind
||扌 double crossed vertical line
||口囗 square form
||乛冖凵 corner to the outside
||八丷 two diverging lines or dots
||小 three diverging lines or dots
- The order of the number is from left upper, right upper, left lower, right lower.
- A long stroke can be broken in two or more parts to obtain the respective numbers, for example:
- 以: the left hook is separated into a vertical (2) and an edge (7), resulting in 2870.
- 习 (simplified character for 習): the hook is separated into a horizontal (1), an edge (7), and a vertical hook to the left (1), resulting in 1712.
- 七: the lower bend is separated into an edge (7) and a bend hook to the right (1), resulting in 4071.
- Upper and lower part of single unit characters can be separated:
- 半 is separated into three dots (9) and double crossed horizontal (5), resulting in number 9050
- 火 is separated in three dots (9) and two diverging lines (8), resulting in number 9080.
- If a stroke on the bottom is inclining to only one side, the other side is given the number 0:
- 飞 (simplified character of 飛): the lower hook is (1), the opposite left side is given (0), resulting in 1201.
- 产 (simplified character of 產): the lower stroke is (2), the opposite right side is given (0), resulting in 0020.
- This rule is only valid for single unit characters. Hence, characters like 張誇
obey the "normal" four corner rule.
- All characters written with including structures like 囗門鬥 would have the same number if the four outside corners would be valid. Therefore, the two top numbers are taken from the outside structure, the two bottom numbers from the inside part:
- 園 is 6023 (60 from the top of the outside box, and 23 from the bottom of the inside part 袁)
- 國 is 6015 (60 from the top of the outside box, and 15 from the bottom of the inside part 或)
- 開 is 7744 (77 from the top of the outside "gate", and 44 from the inner part 开)
- 閉 is 7724 (77 from the top of the outside "gate", and 24 from the inside part 才)
- This rule is only valid for including structures that include the whole character, not only parts of it. Hence, characters like 苗恩睦 do not obey this rule.
- If a stroke, sign or shape is already used for one corner, the opposite corner is given the number 0.
- 王 is 1010 (一; already used=0; 一; already used=0)
- 冬 is 2730 (丿; 乛; 丶; already used=0)
- 之 is 3030 (丶; already used=0; 乀; already used=0)
- 八 is 8000 (八; already used=0; already used=0; already used=0)
- For some characters with different appearances in writing and printing, the numbers are standardized:
- 言 0060 (not 1060), 祖 3721 (not 1721), 戶 3020 (not 1020)
- 住 2021 (not 2521), 路 6116 (not 6716), 比 2271 (not 2171), 反 7223 (not 7123), 禺 6022 (not 6042), 卜 2300 (not 4000)
- Even using the Four Corner System there are often many characters with the same number heading. To solve this problem, an additional fifth number (fuhao 附號) is given that derives from the stroke upside the fourth number (left lower corner).
- 廁 is 0022-0 (no stroke upside the fourth corner)
- 廝 is 0022-1 (from the horizontal stroke of 斤)
- 齊 is 0022-3 (from the bow of 氏)
- 齐 (simplified character of 齊) is 0022-4 (from the crossed lines 乂)
- 方 is 0022-7 (from the corner above 亅)
- Even then, there are still lots of characters with the same five numbers. In this case, the horizontal lines are counted and the characters put in an order related to the number of horizontal lines: 0022-7 方市 have 2, 帝 has 3, 高 has 6, 庸 has 7 horizontal lines.
- For many characters, especially simplier ones, it is not easy to figure out what part is a "corner". In general, the first number is taken from the uppermost part of the character, and the third number from the lowermost part. Second and fourth number are often simply a 0.
- 人 is 8000-0
- 大 is 4080-0
- 方 is 0022-7
- 言 is 0060-1