A Base for a Mukteh Item
1. When a person consciously places a muktzeh item on (or in) a
[non-muktzeh] item. which belongs to him, with the intent that it remain
there at the commencement of Shabbos (1), the permitted article is
considered a base for the forbidden article ("bosis le'davar ha'assur")
[and may not be handled on Shabbos] (2). Even if the muktzeh item is
removed during Shabbos, since it had been placed there for the entire "bein
hash'moshos" period (between sunset and the appearance of three stars on
Friday), it is considered a base for the forbidden article and may not be
handled throughout the entire Shabbos (3), even if one needs to use the
body of the article or the space within which it rests.
(1) The intent has to be that the item remain there for the entire twilight
period ("bein hash'moshos"), between sunset and the appearance of three stars.
(2) The article supporting or containing the muktzeh item, acquires the
same level of muktzeh, as the muktzeh item itself. If the article's
exclusive function is to support or contain the muktzeh item, then it is
considered a "base" to the muktzeh item (and hence, forbidden to be
handled), even where there was no conscious intent to leave the muktzeh
item on (or in) it (for example, a wallet) (See Mishna Berura 309:18).
(3) This is based on the principle that something which was considered
muktzeh for the entire twilight period, remains muktzeh for the entire
Shabbos, even if the cause of the muktzeh prohibition is removed