One point of difference between modern houses and traditional minka is that in most old minka there is no standard size for posts. Even within a single minka every post is unique in its dimensions, even if only slightly, because each was hand-worked and finished from hand-selected timbers, and posts of appropriate dimensions were selected for every location, depending on the size of the load, the span between partitions, and other considerations. This is in contrast to today’s machine-milled posts, which are all essentially ‘fungible’ and of uniform dimensions, typically 90 x 90 or 120 x 120mm.
Nor was the customary post pitch (spacing) of one ken (間) standard throughout time and place. The modern ken is defined as 1,818mm. In older minka, pitches greater than this were relatively common. In regional terms, the difference can be broadly classified into the jōhō-ryū (上方流) or ‘jōhō style’ (Jōhō is an old name for the western region of Honshū centred around the old capital, Kyōto) which employed a larger ken, and the kantō-ryu (関東流), the ‘Kanto style’ (Kantō being the eastern region of Honshū centred around Tōkyō, formerly Edo), which used a smaller ken.
This regional difference persists in modern construction. Today there are three main module systems still in use in Japan: the kyō-ma 京間 or ‘Kyoto ken’ of 1,910mm plus post width, the chū-kyō-ma 中京間 or ‘reduced Kyoto ken’ of 1,820mm plus post width, and the edo-ma 江戸間 ‘Edo ken’ a.k.a inaka-ma 田舎間 ‘country ken’ of 1,820mm. That the chū-kyō-ma and edo-ma seem at first glance to be identical is explained by the fact that the kyō-ma and chū-kyō-ma are ‘double module’ systems, called in Japanese ūchi-hō-sei (内法制, lit. ‘inner system’) or tatami-wari (畳割り, lit. ‘tatami division’), where the first module is measured from inner post face to inner post face, and the second module is the dimension of the post, while the edo-ma is a ‘single module’ system, called in Japanese shin-shin-sei (心々制, lit. ‘heart-heart system’) or hashira-wari (柱割り, lit. ‘post division’), where the ken is measured from post centre to post centre.
As the name tatami-wari suggests, the use of a double module arises from prioritising the uniformity of tatami mat sizes over that of post spacing, and allows for a standardised tatami mat of 1,910mm x 955mm under the kyō-ma system, or 1,820mm x 910mm in the chū-kyō-ma system. In contrast, in a centre-to-centre single module system such as the edo-ma, tatami mats are sized to fit around the fixed post module. Remember that in minka the floor covering does not go from skirting board to skirting board between walls, or flow uninterrupted from room to room at openings, as it does in a western house with floorboards or carpet; there is either a visible ‘tatami stop’ (tatami-yose, 畳寄せ) at the base of the half-timbered walls, or a sill (shikii 敷居) at openings, marking the threshold between rooms, with shallow tracks cut into it to receive the sliding room partitions. Both tatami-yose and shikii are generally of the same width as the posts, i.e. their vertical faces are flush with the faces of the posts, and their upper horizontal surfaces are at finished floor level, i.e. flush with the surface of the tatami.
A renovated minka showing the tracks cut into the floor beams or bearers (ōbiki) and sills (shikii) to take the sliding room partitions, now mostly removed.
Image showing the shikii threshold between two tatami rooms and the tatami-yose at the base of the half-timbered walls.
As an example, consider a room of one ken square. In the edo-ma system, the post spacing in each direction is 1,820mm centre-to-centre. Assuming a post dimension, and therefore a sill width, of 90mm, this means that in a two-mat room, i.e. a room that is 1 ken square, the tatami mat length must be 1,820 - 90 = 1,730mm to fit between the posts, and the width 1,730/2 = 865mm. If 120mm posts are used, this centre-to-centre dimension remains unchanged at 1,820mm, but the tatami mats will now need to be 1,820 - 120 = 1,700mm in length and 1,700/2 = 750mm in width.
A 1 x 1 ken, two-mat room with 90 x 90mm posts on the edo-ma or inaka-ma module.
In a room that is 2 ken square, the corner post spacing in the edo-ma system is simply 1,820 x 2 = 3,640mm centre-to-centre. In this case, the tatami mat length must be (3,640 - 90)/2 = 1,775mm, and the width 887.5mm.
A 2 x 2 ken, eight-mat room with 90 x 90mm posts on the edo-ma or inaka-ma module.
Using the double module chū-kyō-ma system, the tatami mat dimensions are fixed at 1,820 x 910mm regardless of the size of the room, and the posts and sills are fit around them. So in a 1 x 1 ken, two-mat room, the actual post centre-to-centre dimension is the length of the mat (1,820mm) plus half the post dimension at either side, i.e. 1,820 + (2 x 45) = 1,910mm. If 120mm posts are used, the centre-to-centre dimension will become 1,820 + (2 x 60) = 1,940mm.
A 1 x 1 ken, two-mat room with 90 x 90mm posts on the chū-kyō-ma module.
In a 2 ken square, eight-mat room built on the chū-kyō-ma module, with 90mm posts, the corner post spacing is (2 x 1,820) = 3,640mm between internal post faces and (2 x 1,820) + 90 = 3,730mm centre-to-centre.
A 2 x 2 ken, eight-mat room with 90 x 90mm posts on the chū-kyō-ma module.
There have been other regional variations on the length of the primary construction module that have not survived into modern times, among them the 1,760mm + post width double module kaga-ma (加賀間) of the old Kaga province, modern-day Ishikawa Prefecture, in the Hokuriku region of north-central Japan. In general, the regions where tatami mats earliest appeared (from the Kinki region westwards) used the tatami-wari system; in regions where sericulture was practised and other areas where board floors remained common (from the Chūbu region eastwards), the hashira-wari system often predominated. There are also instances of the two different systems being used within the same minka: the double module system in the raised-floor living area of the dwelling, and the single-module in the earthen-floored doma utility area. In the famous silkworm-rearing village of Shirakawa in old Hida Province (now northern Gifu Prefecture), many of the minka have posts in their tatami-floored spaces with a centre-to-centre pitch of 1,850mm, while the board-floored and doma areas of the dwellings show pitches of 1,970mm and 2,120mm.