Kelim, Chapter Twenty-Two, Mishnah Five
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.