Mishnah Kelim 22:6-7

A chair whose middle seat board was removed but the outer ones remained, it is still susceptible to uncleanness. If the outer ones were removed and the middle seat board remained it is also susceptible to uncleanness. Rabbi Shimon says: only if it was a handbreadth wide.

A chair, of which two seat boards were removed, this one next to this one: Rabbi Akiva says: it is susceptible to impurity; And the sages say that it is clean. Rabbi Judah said: so too if the seat boards of a bride's chair were lost, though the receptacle under remained, it is clean, since where the primary function has ceased, the secondary one also ceases.