The Kodansha Kanji Learner's Dictionary

This Revised and Expanded Edition increases the number of kanji entries from 2,230 to 3,002, as well as changing all pronunciations from romaji to kana.

Based on how a kanji can be divided into sections, they are grouped into four main categories: Left-Right (1), Up-Down (2), Enclosure (3), and Solid (4).

Thus, 明 would be in the first main category (Left-Right), would have four strokes in its left segment and four in its right, giving it a SKIP number 1-4-4.

The kanji 測 (measure) can be broken down into three segments 氵 (water), 貝 (shell money), and 刂 (knife).

The Solid SKIP codes put the stroke count second, and the subpattern third, in order to aid in linear searching (such as in the paper dictionary).

In its index for kanji, pages where a user might erroneously expect a kanji to be located (either because of incorrect classification or incorrect stroke count) are cross-referenced with their correct SKIP number.

Since many kanji sharing this radical (such as 開, 3-8-4, or 閉, 3-8-3) follow this 3-8-… pattern, 門 itself is also cross-referenced there.