According to Barth Fruit, the Calamondin orange (citrofortunella microcarpa) is a naturally occurring hybrid of the citrus reticulata (a type of mandarin orange) and the fortunella margarita kumquat (oval kumquat), which is presumed to have originated in China. It is reportedly hardy to 20 degrees F. and is hardier to cold than most other true citrus specie.
The Calamondin orange is cultivated particularly in the Philippines, where it is also known as the Calamansi. The leaves are oval and their underside is much brighter than their dark green upper side. The fruit has very sour taste even when ripe. My folks used the fruit as juice diluted with water and sweetened with sugar or honey. This can be drunk cold or warm for sore throat. The pure juice is also used with soy sauce as a marinade for meat or chicken. Our calamondin tree had to spend most of the winter in the basement, and it lost some of its buds but it is looking gorgeous again!