(a) The RITVA here says that the Gezeirah prohibiting lying on top of ten
sheets that are on top of Kil'ayim is actually a "Gezeirah l'Gezeirah," a
rabbinical decree made to safeguard another decree. The original Gezeirah of
the Rabanan was to prohibit lying directly on a garment of Kil'ayim, lest a
thread of Kil'ayim wrap up onto his skin. The Gezeirah prohibiting lying on
ten sheets which are on top of Kil'ayim was enacted in order to prevent one
from lying directly on Kil'ayim, which would then be Asur because a thread
might wrap onto his skin. (Both Gezeiros are considered to be one Gezeirah -
- "Kula Chada Gezeirah" -- since they were both enacted at one time.)
(b) The RAMBAM (Hilchos Kil'ayim 10:12) appears to have learned that even if
a garment of Kil'ayim is beneath ten sheets, we are still afraid that a
thread from the garment of Kil'ayim might somehow get on top of the sheets
and reach one's skin. It is all the same Gezeirah. This also appears to be
the opinion of TOSFOS (DH Kashim).
The Rambam and Tosfos are consistent with their own opinions. They hold that
even if a garment is very hard, it is prohibited to lie directly on top of
it. It is only permitted if there is something separating between his skin
and the hard garment of Kil'ayim. They consider sitting on Kil'ayim to be a
form of "Ha'ala'ah," placing the garment on one's body, because what
difference does it make if the garment is on top of his body, or his body is
on top of the garment? As long as one's flesh is touching the garment, it is
considered Ha'ala'ah. According to this view, it is not necessary to have a
Gezeirah to prohibit sitting on Kil'ayim because of the reason that a thread
might wrap up onto one's body. Why, then, was such a Gezeirah made? It must
be that the Gezeirah that a thread might wrap up onto his body was made to
prohibit a garment of Kil'ayim underneath several sheets.
(c) The RAN here infers from RASHI (DH Shema Tikarev) that the prohibition
against lying on Kil'ayim lest a thread wrap itself up onto his body is an
Isur d'Oraisa and not d'Rabanan. When the Beraisa says that "the Chachamim
said that it is forbidden [to lie on Kil'ayim]," it means that it is
forbidden because it is a Safek of an Isur d'Oraisa. In the case of the
Kil'ayim underneath ten sheets, there is no Isur d'Oraisa, but only a
Gezeirah so that one not come to sit on the garment of Kil'ayim itself.
(d) The RAN himself, though, suggests that the Gezeirah of lying on top of
sheets which are on top of a garment of Kil'ayim is a Gezeirah to prevent
Ha'ala'ah -- to ensure that one does not pick up the garment and wear it. It
has nothing to do with the fear that a thread will wrap onto one's body; we
are not afraid of that, since the Kil'ayim is below other sheets.
This Gezeirah that one might wear the Kil'ayim, however, applies only to the
type of garment that is normally worn, but not to sheets. If the Kil'ayim on
bottom is a sheet, it is only prohibited to sit directly on top of it,
because there is no fear that one might pick it up and wear it. Rather,
there is only a fear that since he is sitting directly on the Kil'ayim, a
thread might wrap up onto him. An item of clothing, on the other hand, is
prohibited even if it is under ten sheets, because then the Gezeirah that he
might wear it applies.
This explanation is supported by the text of the Beraisa which says, "Even
ten sheets, one on top of the other, and Kil'ayim *beneath them* [is
prohibited]." This implies the prohibited item itself is not a sheet, but is
a different garment made of Kil'ayim.