Kosen judo (高專柔道 Kōsen jūdō?) refers to a set of competition rules of Kodokan judo with particular emphasis on ground grappling techniques such as pinning holds (osaekomi-waza), joint locks (kansetsu-waza) and chokeholds (shime-waza), referred to as newaza in Japanese martial arts. 
Kosen (高専 kōsen?) The Ko in Kosen actually meant High School judo, and based around the Budokuden Martial Arts hall in central Kyoto. Schools started holding their own judo competitions from 1914. The rules of a Kosen judo match were mainly Dai Nippon Butokukai and Kodokan rules prior to 1925 changes. They allowed direct transition to newaza, enabling scenarios where one less skilled judoka could drag down the other into newaza (a tactic now known as pulling-guard), and this was exploited by some teams that matched their less skilled students against the more skilled students of the rival teams, aiming at a draw in newaza. To achieve victory under the judo rules of the time the judoka had to score ippon as there were no intermediate scores, or a draw was declared at the referee's discretion.
1925 changes were largely a reaction to Kosen competition's emphasis on newaza. As opposed to earlier ruleset, transition to groundwork was limited by much stricter rules and by 1929, yusei-gachi rule was introduced to end draws in matches. However, Kosen schools continued to hold interscholastic competition (高專大会 kōsen taikai?) tournaments with former rules.
Differently to modern Judo rules leg-locks were allowed. (Leg-locks started being prohibited by Kodokan rules in 1914 in shiai and randori as well. By 1925 all joint-locks except elbow locks were totally prohibited together with neck cranks. Kosen rules being the Kodokan rules derivative did not allow leg-locks absolutely).
The matches had no time limit and were usually contested on a mat 20x20 meters in total size. A starting zone 8x8 meters was marked on the mat as well as a danger zone which ended at 16x16 Meters.
− If a Judoka went out of the danger zone the match would be restarted. If they were actively engaged in newaza the referee would call sono-mama to freeze them into position, drag them to the middle of the competition area, and call yoshi to restart the match in the same situation. This device was common in Judo in general and is still part of the official Judo rules, addressed in article 18 - Sono-mama, but has since fallen into disuse, allowing modern Judoka to escape newaza by going out of the competition zone.
Kosen judo style
Kosen judo, as a distinct "style" focusing training towards the Kosen ruleset, flourished in the Kyoto region until around 1940. The style and the peculiar ruleset is still studied for "seven imperials judo" (七帝柔道 shichitei jūdō / nanatei jūdō?) tournaments of (former) imperial universities and is taught especially in Kyoto.
- Green, Thomas A.; Svinth, Joseph R. (2003), Martial Arts in the Modern World, Westport, CT: Praeger
- Kashiwazaki, Katsuhiko (1997), Osaekomi, Ippon Books
- Judo History Archive