Arachin, Chapter 9, Mishnah 7. This chapter has been working through the laws of redemption: the ancestral field that was sold, the field bought from another, and the house sold inside a walled city. Each of those categories has its own rules about when the sale can be undone and at what price. Our mishnah now turns to a case that sits between two of those categories: houses that stand in a settlement without the status of a walled city, what the mishnah calls batei hachatzerim, the houses of the courtyards.
Our mishnah lays down a rule of double benefit. Such a dwelling, says the mishnah, is given "notnim lahem" - we assign to it "koach hayafeh", the stronger hand, "shebevatei arei chomah", whatever is advantageous in the halachot governing homes of walled cities, together with "vekoach hayafeh shebasadot", whatever is advantageous in the halachot governing ancestral fields. This building, then, refuses to be filed under a single heading. Whichever of the two bodies of law happens to favor the seller is the one that governs him: sometimes the ruling of the city home, sometimes the ruling of the field. The mishnah proceeds to spell out both halves.
The advantage taken from the walled city
The advantage taken from the walled city
The advantage taken from the field
The advantage taken from the field
So the seller of such a house gains on both sides of the ledger. For the swiftness of redemption we treat his house like a house of a walled city, and for the return at Yovel and the reduced redemption price we treat it like an ancestral field. He receives the better half of each set of laws.
Which houses are these?
"Ve'eilu hein batei hachatzerim: shtei chatzerot shel shnei batim." Here comes the definition, and it is a modest one: a pair of courtyards, with two homes standing in each. And then the surprise: "Af al pi shemukafin chomah", even should a wall enclose them, and not a wall of recent construction but one "mimot Yehoshua bin Nun", going back to the conquest of the land, nevertheless "harei eilu kevatei chatzerim" - their halachah remains that of courtyard homes, with the twofold leniency just described.
Why should a wall from the time of Yehoshua bin Nun not be enough? Because the status of batei arei chomah depends on a city surrounded by a wall, and a cluster of two courtyards is simply not a city. There are not enough inhabitants there to call the place by that name. The wall may be ancient and authentic, but the settlement inside it never rises to the definition the Torah requires, and so the houses keep the leniencies of houses of the courtyards.