Comentario sobre Keilim 7:6

כיצד משערין אותם – because the legs/pins are far from each other, and when an unclean reptile is found between them, we are not able to know if it is within the three [fingers] to the rim/lip and impure, or beyond the three [fingers] to the rim and pure for airspace for Rabbi Meir and for Rabbi Shimon, whether for contact and/or for airspace, therefore, one measures to this leg/pin three fingers from the rim/lip and similarly to the second and placer the cubit and draw [tracing] lines of the boundaries between them from one to the other, and this is called a כנה/putting a ruler between, measuring a straight line (or putting the base of the stone between them) and he draws lines, and from the tracing lines and inward is impure, that is, the place of the straight/ruled line, and from the tracing lines and outward is pure.

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.