Judo derives from Jiu-Jitsu, where all joints in the body (including leg, back, fingers etc) can be locked, to devastating affect. The original Judo allowed for a much wider set of locking techniques than are permitted in the modern sport, in which only Arm-Locks are legitimate techniques.
The 10 standard Kodokan techniques of Kansetsu-Waza comprise 9 arm-locks and one leg-lock (Ashi-Garami). Ashi-Garami is one of the four formally forbidden techniques (Kinshi-Waza), although the study of it is retained through the Katame-No-Kata.
Click on the link below for a great instructional video covering the main techniques.
Then click on any image below, or the button below the image for (different) videos for a specific technique...
A technically challenging 'figure-four' armlock, Ude-Garami places a sideways pressure on the elbow joint in a reverse direction to other typical armlocks. Mastered by the Judo prodigy Masahiko Kimura (in 1935 at the age of 18 he became the youngest ever Godan (5th degree black belt)) : in Brazilian Jiu-Jitsu this technique is known as the 'Kimura'.
Arm Locking/Crushing Straight Control
The 'straight' armlock is rarely used to submit an opponent in sparring (Randori) or competition as it is relatively easily to escape. Ude-Gatame is typically used in a standing position to distract an opponent, or on the ground as a transition to another Kansetsu-Waza, or Osaekomi-Waza (pinning-holding technique).
However, click the image above to watch one particular specialist (Koji Komuro) in competitive action with Ude Gatame!
Arm locking/Crushing Armpit Control
A classic of self-defence, this armlock is executed with Uke facing the floor, standing or on their stomach, while Tori has uke's arm trapped with the elbow beneath Tori's armpit. Tori lifts uke's hand, causing hyperextension of the elbow joint in a similar manner to Ude-Hishigi-Juji-Gatame.
Arm locking/Crushing Triangle Control
Sankaku is a control usually applied with the legs to form a triangle, capturing both the opponent's arm and neck. It can result in a strangle, a hold or in this case a position from which to effect an armlock. See also Sankaku-Jime (Strangle) or Sankaku-Osae-Gatame (Hold).