The Chiang Mai chili is a unique strain to the north. It is something of a combination between the relatively mild banana chili and the spicy spur chili. It has a curved shape like the spur, but the skin is more irregular like a banana chilli. When green, the chili has the lime color of the less spicy chili, but unlike the banana chili the Chiang Mai chili will turn red when fully ripe. However, the Chiang Mai chili is rarely allowed to fully ripen, since the primary use of the chili is to make one of Chiang Mai's most well known dishes, a fiery hot chili dip.
Chiang Mai Chilies
You probably won't be able to find Chiang Mai chilies anywhere outside of Thailand. They're even hard to find in Bangkok. In his book, David Thompson suggests substituting equal amounts of banana and spur chilies. If you can't find either of them, I've used Mexican Serrano chilies to good effect.