Kelim, Chapter Twenty-Two, Mishnah Six
1) A chair whose middle seat board was removed but the outer ones remained, it is still susceptible to uncleanness.
2) If the outer ones were removed and the middle seat board remained it is also susceptible to uncleanness.
a) Rabbi Shimon says: only if it was a handbreadth wide.
Section one: The chair referred to here has three seat boards. If only the middle one is removed, it is still susceptible to impurity. The outer ones should be sufficient to support the sitter’s tuches (the seat on my bike is a little like this).
Section two: So too, if the outer ones are removed and only the inner one remains. The middle one can still be sat upon, so the chair is still susceptible to impurity.
Rabbi Shimon adds that the middle one must be at least one handbreadth wide. If it is not, it is pure.