A board is placed on top of an old oven and protrudes a handbreadth in two direc-tions, but not on the other sides. [If there is] tumah on this side, utensils that are on the other side remain tahor. R’ Yose declares [them] tamei. A b’tach does not spread tumah. If there is a ledge, R’ Eliezer says: It does not spread tumah. R’ Yehoshua says: We view the btach as if it is does not exist, and the ledge above it spreads tumah.