The word dohyo is the name for the fighting arenas of most martial arts/sports. This text will, however, focus on the sumo dohyo.
The word "do-hyo" means "soil" (or "earth") and "bale"1 (of straw). Which is what the dohyo is made up of: a good 30 tons of special clay and sand (from the Ibaraki prefecture, northeast of Tokyo2), and bales of rice straw. It is built from scratch for every tournament by the yobidashi (the ushers or stewards), a 60cm high platform, slightly wider at the bottom. The area on top of the platform measures approximately 540*540cm, and this is where most of the action happens3.
When it has been properly purified and blessed the dohyo is a sacred place. Which, among other things, means that women may not stand upon it! On the Saturday before opening day of Hon Basho (the bi-monthly, 15-day tournaments) a ritual is performed by the highest ranking gyoji4 (referees) and gyoji from makuuchi5 and juryo6. In the middle of the dohyo six items are buried: salt, washed rice, Torreya nuts, squid, kelp, and chestnuts. From this moment the dohyo is pure.
The wrestling takes place in a ring, 455cm in diameter, marked by almost-buried bales of straw (tawara or dawara). The border is known as the shobudawara (actually the "win-lose bales"); overstepping this border after the bout has begun means losing. Just outside the shobudawara is a strip of finely raked or brushed sand: the janomenosuna (the "snake eye ring"). The yobidashi make sure to brush the sand before each bout, to make it easier to see the mark from an errant toe or heel7.
In the middle of the dohyo are the shikirisen - the starting lines. Two white lines, 90cm long and 6cm wide, set 70cm apart. This is where the wrestlers crouch down and face each other, touching their fists to the ground right before tachiai (the initial charge).
On the sides of the dohyo there are built-in steps - fumidawara. Three on each of the sides facing east, south, and west, and one on the northern side (only the north side shimpan (judge) is supposed to climb the dohyo from this side). On the SE and the SW corners are placed the salt boxes and water buckets. The wrestlers use the water to rinse their mouth, and the salt to purify the ring (by throwing it on the ground).
Over the dohyo, hanging from the ceiling, is the tsuriyane; a roof, resembling a Shinto shrine. From each of its corners hang coloured tassels: clockwise from the SE-corner: red, white, black, and green. The tsuriyane is the most visible reminder of the very strong influence Shinto has had on sumo. Even now, in modern day Japan, the rituals around and on the dohyo reflect how deeply sumo is anchored in the history of Japan.
- The Chinese reading of the kanji (俵) is "hyo". In Japanese it is "tawara".
- Traditionally, the clay and sand came from the Arakawa river in Arakida, Saitama Prefecture.
- Some of the action in sumo take the wrestlers flying over the edge, onto the spectators closest to the dohyo.
- The gyoji are ranked as strictly as the wrestlers. Highest ranking gyoji officiates bouts between the highest ranking wrestlers.
- Makuuchi is the top division in sumo.
- Juryo is the second highest division in sumo.
- If the wrestler touches the ground outside the ring, or if he touches down inside the ring with anything other than the soles of his feet - he loses.
My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html
sumoforum.net/glossary.html#H is an awesome source of information.