Quoting%20commentary for Eruvin 3:3

נְתָנוֹ בְאִילָן, לְמַעְלָה מֵעֲשָׂרָה טְפָחִים, אֵין עֵרוּבוֹ עֵרוּב. לְמַטָּה מֵעֲשָׂרָה טְפָחִים, עֵרוּבוֹ עֵרוּב. נְתָנוֹ בְּבוֹר, אֲפִלּוּ עָמוֹק מֵאָה אַמָּה, עֵרוּבוֹ עֵרוּב. נְתָנוֹ בְרֹאשׁ הַקָּנֶה אוֹ בְרֹאשׁ הַקֻּנְדָּס בִּזְמַן שֶׁהוּא תָלוּשׁ וְנָעוּץ, אֲפִלּוּ גָבוֹהַּ מֵאָה אַמָּה, הֲרֵי זֶה עֵרוּב. נְתָנוֹ בְמִגְדָּל וְאָבַד הַמַּפְתֵּחַ, הֲרֵי זֶה עֵרוּב. רַבִּי אֱלִיעֶזֶר אוֹמֵר, אִם אֵינוֹ יוֹדֵעַ שֶׁהַמַּפְתֵּחַ בִּמְקוֹמוֹ, אֵינוֹ עֵרוּב:

If he placed it in a tree [ four by four tefachim or more, standing in a public domain], higher than ten tefachim, his eruv is not valid. [For since the tree is four tefachim wide, higher than ten, it is a private domain, and he acquires habitation in the public domain. So that if he wished to take his eruv and eat it at the time the eruv effects acquisition for him, i.e., ben hashmashoth (at twilight), he would not be permitted to do so, for he would be taking it from a private to a public domain. Therefore, it is not a valid eruv.] Below ten tefachim, his eruv is an eruv. [For below ten tefachim is a karmelith (anything three to nine tefachim high and four wide being a "karmelith.") Taking the eruv, then, would involve only a (rabbinic) interdict of shvuth (resting). Therefore, it is a valid eruv. Our Mishnah is in accordance with Rebbi, who says: Anything which is interdicted by reason of shvuth was not decreed against ben hashmashoth. At the time the eruv effects acquisition for him — ben hashmashoth — he is permitted to take it, so that "he and his eruv are in one spot," for which reason it is a valid eruv.] If he placed it in a hole [in a karmelith, as in a valley or in a field, desiring to acquire habitation in the valley or in the field], even if it were a hundred cubits deep, his eruv is valid. [For the hole itself is a private domain, and he acquires habitation in the karmelith. For at the time the eruv effects acquisition for him — ben hashmashoth — he is permitted to take it. This, in accordance with Rebbi, who says: Anything which is interdicted by reason of shvuth was not decreed against ben hashmashoth.] If he placed it on top of a reed or on top of a pole, which was uprooted and stuck (in the ground) [and which is not four tefachim wide below, in which instance it is not a private domain] — even if it is a hundred feet high, it is a valid eruv, [even though it is four tefachim wide above (for an eruv must be on top of a place of four tefachim.) ("uprooted and stuck":) Only then is it an eruv, but not if it were rooted — a decree, lest when he takes the eruv, he breaks it. For a reed, being soft, is susceptible of being broken. But a tree is hard, and ben hashmashoth there is no apprehension of "lest he go up and tear off (fruits)." But, we do fear that he will lop off the reed and be liable by reason of kotzer ("harvesting"). Or, with reed and pole, there is a possibility that he will confuse what is rooted for what is torn off; for many reeds which are uprooted and stuck in the ground give the appearance of being rooted, for which reason it is to be decreed lest he lop off what is rooted, thinking it to be unrooted. But with a tree, there is basis for a decree of lest he go up and tear off, thinking it to be unrooted.] If he placed it in a closet and lost the key, it is a valid eruv, [as when the lock were tied with ropes in such a manner that if the key were not found, they could be cut only with a knife. The first tanna holds that since it could be opened by cutting the ropes with a knife, there is no skilah (stoning) interdict here, but one of shvuth (resting), the one who opens it "destroying" in the cutting of the ropes, all who thus "destroy" not being liable. And anything interdicted by reason of shvuth was not decreed against ben hashmashoth, as stated above. Therefore, it is a valid eruv; for he can bring a knife, cut the ropes and take the eruv.] R. Eliezer says: If he does not know that the key is in its place it is not a valid eruv. [R. Eliezer holds that an instrument may be handled only for its regular use and that it is forbidden to take a knife to cut ropes with, it being used regularly to cut food with. And since there are two (interdicted acts), the handling of the knife and the cutting of the ropes — even Rebbi, who says that anything forbidden by reason of shvuth was not interdicted ben hashmashoth — even Rebbi concurs that in this instance they did decree. The halachah is not in accordance with R. Eliezer.]

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