What is a Bosis?
- An object upon which Muktzeh was left intentionally from before Shabbat and one intended that it stay there as Shabbat entered (during Bein HaShemashot), the object becomes a Bosis. 
- One may not move a Bosis on Shabbat even if one needs its place or to use it for a permitted use. 
- If money or any other muktzeh item was left unintentionally on a table, one may learn on the table on Shabbat if it isn’t certain that learning there will cause the money to move. 
- If one realizes that he left money in his pocket on Shabbat, one may shake out the money and then wear the clothing. However, if one already is wearing the clothing, he may continue wearing it until he reaches a place where he can shake out the money. 
- If raw meat or another muktzeh item was left on the door of one’s freezer, the door does not become a bosis. 
- A basket of fruit with a rock in it shouldn’t be moved without shaking out the rock or at least spilling out everything and returning the fruit without the rock. However, if spilling/shaking out the basket will ruin the fruit one may move the basket as a whole. 
- A purse which is made to hold money if there’s no money inside is considered a Kli Sh’Melachto LeIssur and so if there’s a key inside one may open it to take out the key. 
- If someone leaves items in a drawer with a muktzeh item on top of a permitted item, some say that the permitted item on bottom isn't muktzeh, while some disagree. Ashkenazim can be lenient in a case of need, Sephardim should be strict.
A bosis with both Muktzeh and non-Muktzeh
- A vessel which has on it both a muktzeh item and a non-muktzeh item is non-muktzeh if the non-muktzeh item is more impotant than the muktzeh item, however, if the muktzeh item is more important, then the vessel is muktzeh.  If they are both equal some poskim hold that it isn't a bosis.
- What is considered more important depends on the owner. 
- Generally the more valuable item is the one which is considered more important. However, many opinions hold that if an non-muktzah object is needed for that Shabbat it is considered more important. 
- Kitzur Shulchan Aruch 89:1
- Kitzur Shulchan Aruch 89:1
- The Mishnah (Shabbat 142b) states that if one wants to move a barrel on Shabbat and a rock was left on top of the barrel from before Shabbat, he may tilt the barrel so that the rock falls off and then move the barrel. The Gemara limits this to a case in which the rock was left unintentionally; however, if the rock was left on top of the barrel intentionally, one may not move the barrel at all, as it is considered a bosis for muktzeh, serving as a support of the muktzeh item. The Mishnah explains if one isn’t able to tilt off the rock, such as if there’s a fear of it breaking other barrels nearby when it is tilting off, one may move the barrel with the rock on top until one reaches a places where one can safely remove the rock. The Rambam (Shabbat 25:17), Tur, and S”A 309:4 codify this as Halacha.
- The Maharil (Shabbat n. 28) writes that one may learn on a table on which money was left because it isn’t a psik reisha that the coins will move. Olat Shabbat 309:2, Ba’eir Heiteiv 309:3, and Kaf HaChaim 309:11 codify this statement. Migdanot Eliyahu 2:69 infers that the case is where the coins didn’t make the table a bosis; otherwise, the Maharil should have been concerned with moving the table and not just the coins. Accordingly, Mishna Brurah (309:15 and Shaar HaTziyun 309:19) explains that had it been a psik reisha that one will move the coins, he would have to tilt off the money unless tilting it off would cause a loss (as in S”A 309:4). Additionally, he adds that the Maharil assumes that a psik reisha of a d’rabanan prohibition is forbidden, unlike the Trumat HaDeshen (see Magen Avraham 314:5).
- The Beit Yosef 309:5 writes that if one forgets money in a pocket which is sewn onto the clothing at the top of the pocket and not along its entire side, the clothing does not become a bosis, as the money isn’t on top of the primary part of the clothing. Nonetheless, adds the Beit Yosef, it is forbidden to wear the clothing because of a prohibition that one may carry it on Shabbat. Rama 310:8 agrees.
- Magen Avraham 310:7 elaborates that regarding a pocket which is sewn only at its top to the clothing, one does not even have to shake out the money before moving it, since it was left there unintentionally and the money is not on the primary part of the clothing. If the pocket was sewn along the entire length onto the clothing, however, the money would need to be shaken out. Mishna Brurah 310:29 writes that if the money was left there intentionally, one shouldn’t put his hands in the pocket because the pocket itself certainly is a bosis. Rabbi Yisrael Bodner (Tiltulei Shabbat p. 94) writes that the common pants pockets are sewn only at the top of the pocket, whereas shirt pockets are sewn along their entire length. He notes that in either case, if one wants to wear the clothing he must shake out the money.
- Shemirat Shabbat KeHilchata 20:75 writes that if one realizes that one forgot money in his pocket only after he already is wearing the clothing and would be embarrassed to remove the clothing in order to shake out the muktzeh, he may continue to wear the clothing until he reaches a place where he can shake out the muktzeh.
- The Chayei Adam 67:10 writes that if only a few insignificant coins were left in the pocket, they are nullified to the clothing because a person wouldn’t nullify the use of his clothing because of a few coins. Mishna Brurah 310:31 agrees.
- The Gemara (Shabbat 120b) records the opinion of Rav, who forbade opening or closing a door with an oil candle built in. Tosfot (s.v. Pote’ach) explain that when the door is moved, the oil in the candle is distanced or brought closer to the wick, thereby violating Mechabeh or Mav’ir. Tosfot ask why the Gemara didn’t forbid it because the door is bosis. Their first answer is that the door can’t become a bosis, as it is nullified to the house and not to the candle. Mordechai (Siman 400) agrees.
- Although the Bei’ur Halacha s.v. Af notes that the Yerushalmi considers the door a bosis unless the candle was left there unintentionally, he suggests that the Yerushalmi is in disagreement with the Bavli. Mishna Brurah 277:4 and Kaf HaChaim 227:11 quote the Mordechai as halacha. Thus, Shemirat Shabbat Kehilchata 20:77 writes that if muktzeh is left hanging on a house or cabinet door, the door does not become a bosis. Rabbi Hershel Schachter (Halachipedia Article 5773 #22) said that the same would apply to frozen raw meat or other muktzeh items on the door of his freezer, and the door does not become a bosis.
- S”A 309:3, Mishna Brurah 309:10
- Shemirat Shabbat KeHilchata 20:16
- Trumat Hadeshen Pesakim 193 writes that some wanted to permit something that was left haphazardly on top of something else unless it was left there to rest there securely. He disagrees with that position. Magen Avraham 309:6 accepts the lenient view. Rabbi Akiva Eiger 309 rejects one of the proofs of the Magen Avraham.
- Mishna Brurah 309:18
- Chazon Ovadia v. 3 p. 133
- The Shulchan Aruch 310:8 writes that an object which has on it both a mutkzah item and a non-Muktzeh item is permissible to move if the non-Muktzeh item is more important (חשוב).
- Chazon Ovadia v. 3 p. 131 writes that the Chaye Adam 67:8 holds it is forbidden if both are of equal importance against the Aruch Hashulchan 310:15 who is lenient. He sides with those who are lenient because of the Ran Shabbat 47a and Hagahot Maimoniyot 26:7 who imply it is permitted.
- Mishna Brurah 310:33 writes that it only depends on the intent of the owner and not the general convention of most people.
- Halachos of Muktzah (by Rabbi Bodner, pg 309, answers of Rav Moshe Feinstein #31, to see it inside click here) writes that if a siddur and money are on a vessel the siddur is considered more important, and so too if one has glasses which one needs immediately and a lot of money, the glasses are more important unless one has other glasses in which case the money is more important. Lastly if a loaf of challah and money are on a vessel the challah is more important since you’re going to need it on Shabbat. This is also the opinion of the Shemirat Shabbat KeHilchata 20:61 in name of Rav Shlomo Zalman that anything which is needed for that Shabbat such as challah or something else of value. [See also Chaye Adam (Shabbat 66:4) and Magen Avraham 279:5 who seem to hold of this.]