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In Keilim 27:3, we see the rule for when a clothes item becomes susceptible to tumah when it's made of different materials, each with a different minimum size to be susceptible to tumah.

הָעוֹשֶׂה שְׁנַיִם מִן הַבֶּגֶד וְאֶחָד מִן הַשַּׂק, שְׁלָשְׁתָּן מִן הַשַּׂק וְאֶחָד מִן הָעוֹר, אַרְבָּעָה מִן הָעוֹר וְאֶחָד מִן הַמַּפָּץ, טָהוֹר. חֲמִשָּׁה מִן הַמַּפָּץ וְאֶחָד מִן הָעוֹר, אַרְבָּעָה מִן הָעוֹר וְאֶחָד מִן הַשַּׂק, שְׁלֹשָׁה מִן הַשַּׂק וְאֶחָד מִן הַבֶּגֶד, טָמֵא. זֶה הַכְּלָל, כֹּל שֶׁחִבֵּר לוֹ מִן הֶחָמוּר מִמֶּנּוּ, טָמֵא. מִן הַקַּל מִמֶּנּוּ, טָהוֹר:

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. 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. 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. [from Sefaria]

This follows up 27:2, where we learn that cloth (בגד) becomes susceptible to tumah at a size of 3 handbreaths x 3 handbreaths, sack-cloth (שק) at 4x4, leather (עור) at 5x5, and matting (מפץ) at 6x6. In our mishnah (27:3), we learn that a more stringent material can help make up the minimum size of a less stringent material, but not the other way around.

The Bartenura's commentary, however, limits this to the combinations mentioned in the mishnah, which are combinations of adjacent materials in the list of cloth, sack-cloth, leather, matting.

ודוקא כסדר הזה הכתוב במשנה מצטרפין לקל שבהן, הבגד והשק, השק והעור, העור והמפץ. אבל בגד ועור, או בגד ומפץ, או שק ומפץ, אין מצטרפין, דהמחברן בטלה דעתו אצל כל אדם. ומיהו לעשות מהן טלאי על גבי מרדעת של חמור, כולן מצטרפין, דמאיזה מהן שיקצע טפח ויעשה אותו טלאי למרדעת של חמור, מצטרף, דאינו מקפיד לשם אם הוא משני מינים:

And they combine specifically in the order mentioned in the mishnah... but [the other combinations] do not combine, since the intention of the one who joins them is irrelevant in the face of people's usual intentions (בטלה דעתו אצל כל אדם). But when making a covering for a donkey's saddle, they all combine, since one can cut off any handbreath [squared] piece from the [combined] material and make a covering for a donkey's saddle, it combines since one doesn't mind if [the material] is of two different types.

I don't understand the reasoning for limiting the combinations. We might infer from the last sentence that people don't care if there are two types of material in their saddle coverings that people do care (in the first clause) if their other cloth-like objects are of two materials; but then why do even two adjacent materials combine, if the result is of two different materials? What is the Bartenura's source for his explanation?

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