5. An object does not become a base [for a muktzeh article] unless the
latter was placed on it with the intention that it remain there for the
duration of the "bein hash'moshos" ("twilight") period (1). In contrast,
if it was inadvertently left there or fell on it accidentally, the object
is not considered a base (2).
(1) This refers to the period between sunset and the appearance of three
stars, during which we refrain from doing activities which are prohibited
on Shabbos, due to our inability to determine the precise moment that
"night" (and therefore, shabbos), begins. In order for the object to become
a base for the muktzeh, one has to have intended to leave the muktzeh item
on it (or in it) for the entire "bein hashmashos" period (see Mishna Berura
There are those who rule that if one intended to leave the muktzeh object
on top of the non-muktzeh item for the entire "bein hashmashos," but
intended to remove the muktzeh at some stage during Shabbos (either through
a Gentile, or by shaking it off), then the non-muktzeh item does not become
a base for the muktzeh item (and thus may be moved once the muktzeh is no
longer resting upon it). According to this opinion, only if one intended to
leave the muktzeh there for the entire Shabbos could the item it is resting
upon become a base. The Mishna Berura (309:21) rules that one may rely on
this opinion if a loss will result.
(2) This is true even if an individual placed a muktzeh object on a
non-muktzeh object without thinking about removing it prior to the onset of
Shabbos, and it was left there (in other words, there was no intent to
remove before Shabbos, nor was there intent to leave it there during Shabbos).