Kelim, Chapter Twenty-Seven, Mishnah Three
1) If one made up a piece of material from two handbreadths of cloth and one of sack-cloth, or of three of sack-cloth and one of leather or four of leather and one of matting, it is not susceptible to uncleanness.
2) If the piece of material was made up of five handbreadths of matting and one of leather or four of leather and one of sack-cloth, or three of sack-cloth and one of cloth it is susceptible to uncleanness.
3) This is the general rule: if the material added is subject to greater restrictions it is susceptible to uncleanness, but if the material added was subject to lesser restrictions it is not susceptible.
Section one: In this section, the piece added is subject to lesser restrictions, and therefore the entire piece is not susceptible to impurity. For instance, cloth needs to be three handbreadths to be susceptible (greater restrictions) and sack-cloth must be four (lesser restrictions). If he takes two pieces of cloth and adds one piece of sack-cloth, it is not susceptible, even though there are three handbreadths altogether.
Section two: In contrast, matting must be six handbreadths (lesser restrictions) but leather only five (greater restrictions). So if he takes one handbreadth of leather and adds it to five handbreadths of matting, it is susceptible.
Section three: This is the general rule that explains the halakhot in sections one and two.