Mishnayos Oholos Perek 15 Mishnah 2

טבליות של עץ – that are adjacent to one another and each of them are a handbreadth above the ground and touch at their corners – each other with its neighbor that is near it, and it is the manner of the corners of the boards that they are not a handbreadth wide, therefore, vessels that are underneath the first that the defilement is under it are impure, but that which is under the second are pure.

הנוגע בשניה טמא טומאת שבעה – and even though it is taught in the Mishnah in the first chapter [Mishnah 2] that: “Utensils which touch the corpse and utensils [which touch other] utensils are unclean with the uncleanness of seven [days]. The third, whether man or utensils, is unclean with the uncleanness [that passes at] evening.” But it is taught in the Mishnah there (i.e., at the conclusion of Mishnah 3): “The tent does not come under consideration.” Therefore, the board that the defilement is underneath it is not part of the count (i.e., is not included).

עד שיהא בו ריבוע בפותח טפח (until it has a squared handbreadth) – it is a kind of breached/broken-through chest that has four walls that they would make for a table when the table sits upon it, and that breached chest is called a square. And sometimes, that the table protrudes and extends outside from that square a handbreadth or less or more, and now it states that it needs that the table protrudes outward from that square with an opening of handbreadth, for just as what is within the square does not combine, if defilement is also within the square from the inside, the square interposes and prevents the defilement from passing to the outside.