Dō (胴)

Dō is the protective breastplate worn during Kendo, Naginata, Jūkendō, and Tankendo. It is used as a target area for the torso. In its modern form, the shape of the dō curves around the lower torso to deflect blows away from the abdomen. Historically, designs were more cylindrical, fitting more tightly to the shape of the body.

Dō (n.) 1. Abdomen and chest protecting armor used in Kendō. 2. One of the targets in Kendō competition. 3. A strike to the Dō of an opponent.

Construction
Traditional Dō dai were constructed out of bamboo slats. Some were even flexible and could be rolled up for carrying. Dō are generally categorized into 50 take (50pc) and 60 take (60pc) styles, which refers to how many slats are used in its construction. Many modern dō are made of molded plastic, and have proven to be light and durable.

Do mune are made out of leather and usually feature different kazari designs.

Wear
The dō is put on after the ''tare. Similarly the dō must be put on while in seiza. The dō is supported by two longer himo, each which cross the back, go over the shoulders, and fasten to the opposing chichikawa''. Two shorter himo are tied in a bow in the small of the back to keep the dō in place. The dō should cover the tare obi without obstructing the odare, if worn correctly.

Paul Shin talks about how to tie dō himo

The two valid target areas are the right and left sides of the dō.  Formerly, the dō mune was a valid target area for tsuki, however this is no longer the case as a result of the US Occupation after WWII. (Is it valid in Naginata? O_o)