Machshirin, Chapter 2, Mishnah 11. Our masechta is devoted to the liquids that render food susceptible to tumah, but the second chapter has taken us through a series of cases of doubt that are resolved by following the majority. This final mishnah of the chapter applies that same tool of rov to a completely different area of halachah: a pile of produce that is a mixture of two different years of the shmita cycle.
The mishnah opens: "Peiros sheniyah sherabu al shel shelishis", produce of the second year that is mixed with produce of the third year, the second year forming the larger portion. To appreciate why this matters we need to review the pattern of the tithes across the seven year cycle. In the first, second, fourth and fifth years of the shmita cycle the second tithe is ma'aser sheni, which the owner brings to Yerushalayim (or redeems onto money that is spent there). In the third and sixth years that tithe is instead ma'aser ani, given to the poor. And in the seventh year, shevi'is itself, the produce is ownerless and the obligations of tithing do not apply to it at all.
The mishnah proceeds through the various combinations. Produce of the second year blended into produce of the third joins a crop whose tithe is ma'aser sheni to a crop whose tithe belongs to the poor. "O shel shelishis al shel revi'is", or produce of the third year mixed with that of the fourth, where once again one crop owes its tithe to aniyim while the other owes ma'aser sheni. "Veshel revi'is veshel chamishis", the fourth together with the fifth, two years whose tithing obligation is one and the same, ma'aser sheni in both. "Veshel chamishis al shel shishis", the fifth mixed with the sixth, pairing ma'aser sheni yet again with a crop destined for the poor. "Shel shishis al shel shevi'is", the sixth year, whose tithe is ma'aser ani, combined with shevi'is produce, which bears no tithing obligation whatsoever and carries kedushas shevi'is. And "shel shevi'is al shel motza'ei shevi'is", shemitah produce mingled with the crop of the year that follows it.
Why can the owner not simply tithe the pile as it stands? Because there is a governing rule that one may not separate the tithes of one year's crop on behalf of another year's crop. Each year's produce must be tithed from itself. Once two years are mingled together, it becomes essential to determine which year we are dealing with.
The mishnah's ruling is "holchin achar harov", we follow the majority. Whichever year makes up the greater part of the pile determines the status of the whole. If the majority is from a ma'aser sheni year, ma'aser sheni is separated; if the majority is from a ma'aser ani year, ma'aser ani is separated; and if the majority is shevi'is produce, nothing needs to be taken at all.
The mishnah then adds: "mechtzah lemechtzah lehachmir", if the mixture is half and half, so that there is no majority to follow, we rule stringently. What does that stringency consist of? Where the doubt involves shevi'is produce, the owner must satisfy both possibilities: he separates the tithes, since the produce may not be shevi'is at all, and he also treats the produce with kedushas shevi'is, since it may indeed be shevi'is.
When the uncertainty lies between a year of ma'aser sheni and a year of ma'aser ani, the Rav spells out the procedure. The owner sets aside the required quantity and declares it ma'aser sheni, stipulating (al tnai) that if the produce in fact belongs to a year of the poor man's tithe, the portion should take effect as ma'aser ani instead. He redeems it onto coins, brings that money up to Yerushalayim, buys food with it there, and hands the food over to poor people to be eaten within the city. Both duties are thereby discharged in a single act: the value of the tithe was consumed in Yerushalayim, as ma'aser sheni demands, and it came into the possession of aniyim, as ma'aser ani demands. No possibility is left uncovered, and that is exactly the stringency the mishnah calls for.
With this, the second chapter of Machshirin comes to a close.