Keilim, Chapter 28, Mishnah 7. The subject of this chapter is fabric: swatches of material and finished garments, and what determines whether they can take on tumah. Among the yesodot already established is that a scrap of material is treated as negligible until it reaches a minimum area. For tumat midras, the tumah transmitted when a zav or a niddah steps on an object or sits upon it, that minimum is a square of three etzbaot in each direction, roughly three fingerbreadths across and three fingerbreadths down. The Mishnah before us presents a machloket over the way that square is to be measured.
Rabbi Shimon's position is that the square of three by three which Chachamim fixed is to be taken "chutz min hamelal", excluding the melal, meaning the hem. Then, as today, weavers and tailors would take the unfinished border of a piece of material, turn it back upon itself, and stitch it in place so the threads would not unravel. According to Rabhi Shimon, that turned-back border plays no part in the reckoning. What counts is the flat expanse that remains spread out and serviceable, so the material must present three etzbaot by three etzbaot over and above whatever is consumed by the hem.
Chachamim rule otherwise: the three by three is taken "mechuvanot", precisely as the piece presents itself. True, a portion of the border is going to be turned back and stitched, leaving the finished article spread over somewhat less area than the swatch originally showed. Even so, that border is counted in. Material whose dimensions come to exactly three etzbaot by three etzbaot has the shiur for tumat midras, and whatever surface will later be swallowed up by the hem does not disqualify it.
Where the two opinions part company in practice is a swatch whose dimensions come to no more than three by three and which has a hem stitched around its edges. For Rabbi Shimon such a piece has not reached the shiur at all and stays tahor; for Chachamim it possesses the required measure and can receive tumat midras.