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Moving a pen or pencil on Shabbos:
An inexpensive pen or pencil that one is not particular with: Muktzah contains various categories, each with their pertinent laws or moving regulations. A typical inexpensive pen or pencil which one is not particular against using it for other purposes due to fear of damage, is defined as a Keli Shemilachto Le’issur. Following the bylaws of this category, this means that a regular pen may not be moved for the sake of saving it from damage [i.e. Liatzmo], although may be moved for the sake of doing a permitted use with it [i.e. Legufo], or for the sake of using its space [i.e. Limikomo].
An expensive pen or any pen or pencil that you are particular with: An expensive pen which one is particular to not use for purposes other than writing due to fear of damage, is defined as Muktzah Machmas Chisaron Kis, and may not be moved for any purpose on Shabbos with one’s hands. [The same applies for any pen or pencil that one is particular against using for other purposes lest it become damaged, even if the pen or pencil is inexpensive.]
May one use an inexpensive pen or pencil to itch his body?
May one move a pen for the sake of preventing a child from coloring with it on Shabbos?
It is best to do so only with a Shinuiy, such as with one’s feet, or the back of one’s hand. Alternatively, one may move the pen regularly and then make a use of the space that it was sitting on.
 Kalkeles Shabbos of Tiferes Yisrael, brought in Ketzos Hashulchan 108 footnote 13; Az Nidbaru 11:21 p. 38 in his conclusion; Piskeiy Teshuvos 308:10
Other opinions: Some Poskim rule that all pens and pencils are Muktzah on Shabbos due to the ink/led which has no use on Shabbos and is not defined as a vessel. [See Keztos Hashulchan ibid in name of Minchas Shabbos] See also Piskeiy Teshuvos ibid who writes “Practically it is not befitting to be lenient due to Zeilusa Deshabbaso”
Definition of Keli Shemilachto LeIssur: A vessel which is designated for a use that is forbidden to be done on Shabbos [and one is not particular against using it for another use, is defined as MM”I]. [Admur 308:12; M”A 308:5; M”B 308:10] All items that are defined as MM”I retain the laws of MM”I, even if the object is not fit for a use other than its Shabbos forbidden use, if one is not particular against finding a permitted use for the object, such as to cover an item or to scratch one’s back and the like. [Admur 308:85; So is also proven from the fact that candles are not Muktzah Gamur, as brought in Admur 308:33] The reason for this is because the mere fact that one is not particular against using the object for another use itself leaves it on his mind. Thus, since one is not particular against using the MMI to scratch his back it is therefore not off his mind, as perhaps he may decide to use it for that purpose if he so chooses.
 Admur 308:12; Michaber 308:3
 Minchas Shabbos, brought in Ketzos Hashulchan 108 footnote 13; Az Nidbaru ibid; Piskeiy Teshuvos ibid
 Admur 308:4
 Ketzos Hashulchan ibid; See Toras Shabbos 308 footnote 2; Az Nidbaru 11:21; Piskeiy Teshuvos 308:1;
 As it is permitted to use MM”I for a permitted purpose on Shabbos.
 As people who are not knowledgeable of the laws may come to transgress with other Muktzah as well.
 As the purpose of this moving is neither Legufo or Limikomo, and if anything is Liatzmo, to prevent damage. Now, it is questionable whether one may move a Keli Shemilachto Liisur for the sake of preventing damage of either items. [See Az Nidbaru 11:21 p. 41; Rav SZ”A in SSH”K 20 footnote 30] Therefore, it is best to move it with a Shinuiy which is allowed according to all.
 Admur 308:12; M”A 308:8; M”B 308:16