Kelim, Chapter Seven, Mishnah Six
How do we measure them?
Rabban Shimon ben Gamaliel says: he puts the measuring-rod between them, and any part that is outside the measuring-rod is clean while any part inside the measuring-rod, including the place of the measuring-rod itself, is unclean.
Today’s mishnah asks how we measure the three fingerbreadths separating the stove from the three props. The prongs themselves can simply be measured from the edge of the stove, but we need to figure out what is considered to be inside the airspace of the area that is not directly corresponding to the prongs.
Rabban Shimon ben Gamaliel says that he uses a measuring rod to form a straight line from prong to prong, thereby creating a triangle around the stove. Anything outside of this triangle is not susceptible to impurity, and anything inside the triangle is susceptible.