(tarte777) - Rilakkuma kotatsu set 3
Kotatsu is a Japanese style table, basically floor table with a heater under it. On a cold day, I don't know anything more useful. well, maybe I am thinking in this way because I am editing this post when I am sick and the heat looks really charming right now.

Anyway, let's continue... in the old time's kotatsu also known as  Hori-gotatsu. That comes from; hori – digging, ko- fire, tatsu – foot heater. 

Kotatsu style

This invention is based on very old times. it's discovered when people are searching a way to both getting warm and cook at the same time. in time the invention becomes the current style kotatsu.  
There are two different kinds of kotatsu which are used in common.

Charcoal Kotatsu

This one is an old style. to use a kotatsu like this there should be a 40 cm hole on the floor. and its stick to the table frame.

Electric Kotatsu 
This one is started to be used in the 20th century. a modern version. it's used with an electric heater. Kotatsu is used on a futon, a Japanese mattress, and another thick futon is put on it. so between futons, the air will be hot.

kotatsu disassembled

There are interesting kotatsu varieties nowadays, I think one of the most interesting one is the wearable kotatsu.