מסכת כלים

פרק כג משנה ב

The tumah of merkav is a type of tumas midras which applies to items that are ridden upon; as will be addressed in the future, it has different rules than the other types of tumas midras.  There is a difference of opinion as to exactly what fall into this category.  Rashi and others say that it applies to items that would assist one while riding, such as a saddle's pommel (the upward curving or projecting part of a saddle in front of the rider) or cantle (the raised, curved part of the saddle at the back of the rider); this opinion holds that the seat of the saddle itself is subject to tumas moshav, as the rider sits directly on that.  Rambam and others hold that tumas midras applies to items that are straddled, and according to their opinion the pommel, cantle and the seat of the saddle are all subject to tumas merkav.

אלו טמאין משום מרכב

These are tamei as merkav:

The Ashkelonian strap (according to those who follow Rashi's opinion, this refers to a girth strap, which secured the saddle to the animal by passing under its belly; it was a separate component from the saddle itself and attached to it using a pair of rings; according to those who hold like Rambam, this refers to a wide strap that functioned as a saddle);

and a Median mortar (the Median mortar was a receptacle carved into the middle of a long block of wood in which ingredients were crushed or ground by users straddling it, or 'riding' it, at either end; according to some who follow Rashi's opinion, the mishnah here refers to footrests for those straddling the mortar, while others say that it actually refers to the stirrups of a regular saddle, to which a rider's foot applied pressure akin to a pestle in a mortar);

and a camel's packsaddle (the packsaddle was sometimes ridden by people as well; according to those that follow Rashi's opinion, this refers either to handholds that protruded from it, or to a saddle pad that went under the actual packsaddle and helped provide a flatter surface for it);

and a saddle of a horse (again, those who follow Rashi says that this refers to the stirrups);

Rabbi Yosi says,

also, the saddle of a horse (or, according to Rashi, the stirrups)

is tamei because of moshav (the tumah of a seat)

as they stand on it on the sports field (either this means that it is placed on the ground and used as a seat, or that during these sports, riders would not merely straddle the saddle in the usual manner, but would actually stand on it or a stirrup);

but a saddle (or, following Rashi's opinion, the stirrups of the saddle) of a naka (a particularly ornery species of camel)

is tamei (this is either a concession by R' Yosi that such a saddle was only ridden upon in the ordinary manner and is only subject to tumas midras, or is a separate statement of the mishnah's tanna kamma who is conceding to R' Yosi that this saddle is used as a regular seat, and so is also subject to the tumah of moshav).

מה בין מרכב למושב

What is the difference between [the tumah of] merkav and [the tumah of] mishkav?

Merkav,

touching it (which renders the person making contact tamei, but not their clothing) is different than carrying it (in which case both the person carrying the item with this tumah and their clothing would become tamei);

and moshav,

there is no difference between contact and carrying (which both render the person and his or her clothing tamei).

A donkey's packsaddle (a ladder-like wooden frame that sits on the donkey's back)

that one (who is a zav) sits on

is tahor (as the parts of the framer are too far apart to serve as a comfortable seat; others say because this would interfere with its primary function of carrying cargo);

if one changed the number of holes (if holes of for the "rungs" of this frame are drilled so that they are closer together)

or if one opened the hole one into the other (creating a slot in which the "rungs" can be slid along the "ladder", so that they can be closer to each other, or so that a flat board can be inserted into them)

it is tamei (as it can now be used as a more comfortable seat, or as a seat and to carry cargo).