Comentario sobre Keilim 22:5

כסא שלא היו חפויו יוצאין – that the boards/planks do not protrude outside of the chair. That the chair is made from four [pieces of] wood and planks fixed from three directions of the chair stands high under the wood that is fastened with each other, and from above the wood, tablets are made to sit upon them. But if the tablets are broken that are made for sitting, one can tip the chair to each side that one wishes and to sit upon those boards that are affixed from the three sides of the chair, but when they protrude outward, he is not able to tip it on its side. And the covers/seat boards mentioned here are not like the covers/seat boards mentioned in the first part of the Mishnah, for they are like protruding teeth, but rather for the boards that cover the wood are called חפויו/upholstered seat.

A chair whose seat boards did not project and then they were removed, it is still susceptible to uncleanness, for it is usual to turn it on its side and to sit on it. The seat boards of this chair did not project outside of the frame of the chair. Thus this type of stool could be turned on its side, and one could sit on the boards that were used to form the sides. If the seat boards are removed from the place where the chair is usually sat upon, the chair can still become impure, because even before they were removed, it was common to use the chair from the side.