Keilim, Chapter 24, Mishnah 6. Our chapter presents a long series of rulings built on a single pattern: a certain type of utensil is named, and the Mishnah teaches that it comes in three forms, each with its own status regarding tum'ah. Here the subject is the basis, the stand: a low platform or base used either to support a person or to hold objects.
The Mishnah begins with the words "Shalosh basisayot hein": stands come in three varieties.
The first case: "shelifnei hamitah veshelifnei sofrim", and of it the Mishnah rules "temei'ah midras". A base positioned at the foot of a bed, or one placed at the desk where a sofer, a scribe, does his writing, is susceptible to tum'at midras. The reason lies in its purpose: such a base is built so that a person will stand or rest his weight on it. It serves as a step or a footrest in the ordinary course of the day, whether someone is getting up onto his bed or seated at his writing desk. Because people support themselves upon it, it belongs among those objects that a zav or a niddah renders tamei through pressure.
The second case: "veshel delufki, temei'ah tum'at meit." A delufki was a small side table on which the plates, the food and the assorted vessels were laid out and prepared before being carried to the table at which the meal itself was eaten. A base belonging to such a side table can receive corpse tum'ah, since it performs a real service: food and eating vessels rest upon it, which makes it a genuine keli capable of contracting tum'ah. Midras tum'ah, though, has no application here, for no one ever intended this base for sitting. Being sat upon has nothing to do with the work it does.
The third case: "veshel migdal, tehorah miklum." A migdal is a large storage cabinet, and the base underneath it remains tahor from tum'ah of every kind. Why? Because this base is not a utensil standing on its own. It belongs to the migdal, serving it as one of its parts, and a cabinet of that size is itself beyond the reach of tum'ah. The base is swallowed up in the larger object it holds, so its status follows that of the cabinet: neither one is capable of becoming tamei.
The Mishnah has thus drawn three lines with one stroke. A stand made for a person to step on is susceptible even to midras. A stand made to hold food and vessels is a keli that receives corpse tum'ah, but not midras. And a stand that is merely part of an oversized cabinet is not a keli in its own right and remains completely tahor.