“Question: If a makeshift shtender is created by attaching a plank of wood to a tree using two wooden support beams (that connect the shtender to the tree) can it be used on Shabbos?
Discussion: First of all, the Shulchan Aruch (OC 336:11) tells us that it is prohibited to use tzidei ho’ilan on Shabbos, whereas tzidei tz’dodin is permitted to use. This means that an item attached to a tree itself (which is termed tzidei ho’ilan) may not be used on Shabbos, and an item attached to this (termed tzidei tz’dodin) can be used on Shabbos. For example, a washing line or hammock tied directly to a tree are termed tzidei ho’ilan and may not be used on Shabbos; if they are hung on a hook that is screwed into a tree they are termed tzidei tz’dodin and thus permitted to use on Shabbos.
Regarding the scenario at hand; although it could be argued that that the wood that connects the shtender to the tree is itself tzidei ho’ilan, and the shtender connected to this would be tzidei tz’dodin and thus permitted to use on Shabbos, it appears that this contraption is more problematic:
This is because the makeshift shtender is one unit, and the wooden connecting beams have no independent function or use and therefore the whole contraption is viewed collectively as tzidei ho’ilan and forbidden.
The precedent to this is in Siman 305 se’if 18 where we find a virtually identical prohibition regarding using an animal on Shabbos, and the same distinction between tz’dodim and tzidei tz’dodin. I.e. it is prohibited to use an item directly attached to an animal on Shabbos, but it is permitted to use an item attached to this first item. For example, if a wagon is attached to an animal and a second independent wagon is attached to the first, the first wagon directly attached to the animal is tz’dodim and forbidden; the second is tzidei tz’dodin and thus permitted (regarding this prohibition of using an animal; there may still be a prohibition of riding in a wagon in case one comes to break off a branch in order to lead the animal, see Remo and Mishna Berura ibid; also Bi’ur Halocho d.h. “gam”).
The Mishna Berura (ibid s.k. 67) writes (discussing the tailboard of a wagon) that when the components form one unit, the entire contraption is termed tz’dodim and forbidden. He also writes in Sha’ar Hatziyun (s.k. 60) that this is why it is forbidden to climb on a ladder that is leaning against a tree, because even though it can be suggested that only the vertical poles of the ladder are considered tz’dodim, and the rungs would therefore be termed tzidei tz’dodin (and permitted to be used on Shabbos), we see clearly from the Rishonim that this is in fact forbidden. The reason must be because the components of the ladder are viewed as one entity, and therefore the ladder as a whole is termed tz’dodim and forbidden to be used.
It is also implied in the Remo in this se’if (see also Mishna Berura ibid) that the components that harness the animal to the wagon are not viewed as independent entities, and therefore the wagon as a whole is classified tz’dodim and forbidden to use on Shabbos.
Similarly, over here the connecting pieces of wood aren’t viewed as a separate entity to the shtender, rather as a component of the shtender and it would therefore be forbidden to use the shtender as it is termed tzidei ho’ilan.
Conclusion: A shtender created by attaching a plank of wood to a tree is forbidden to use on Shabbos, because of the prohibition of using a tree on Shabbos.
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