Kōzu can be loosely translated as “illustration of fragrances”. The Japanese term comes from a sophisticated parlour game called Genjikō, in which five specimens of five different incense fragrances are prepared, for a total of 25 incense sticks. One player will select five sticks at random, and burn them in order. The other players are then asked to guess which incense fragrances were burned, specifically which, if any, of the fragrances are the same. There are 52 possible combinations, all named after a chapter of the famous 11th Century Japanese novel Tale of Genji (except the first and last chapters), and each combination is represented by a pattern of five vertical bars joined as appropriate at the top by horizontal bars. The players would select one of the patterns indicating which of the fragrances were the same.

If, for example, the third and fifth fragrance were the same and all others different, players would select (numbers are from right to left), which is Chapter 21 Otome (The Maiden).
With a great deal of imagination, you can see how this particular set of kumiko patterns was given this name.

Kōzu pattern

Kōzu pattern
Kōzu pattern

Kōzu pattern

Hitoe-kōzu (with bottom asa-no-ha band)

This is the single kōzu — single because there's only a single set of mitred joints above/below each of the main horizontal kumiko.

Futae-kōzu (Book 2)

This is the double kōzu, and one of my favourite patterns; double because there are two sets of mitre joints above/below each of the main horizontal kumiko.

This pattern is used in shoin-shōji and windows etc., and to me the highlight and main feature of beauty of this design is the wonderful open space formed by and surrounding each of the crosses.

Mage-kōzu (Book 2)

Mageru is the Japanese word for bend, and instead of normal mitre joints, the mage-kōzu, or round kōzu, bends through a series of kerfs cut at the correct positions in the kumiko. This is an extremely difficult pattern to make, as the radius of the bend has to be accurately calculated to ensure that all the normal half-lap housing joints are in exactly the correct position, and that the kōzu loop joins up exactly in the middle of a half-lap joint so that this join is unseen.

The photo shows a
mage-kōzu I made as a trial piece at Shokugei Gakuin.