1) HALACHAH: SITTING ON A GRAVESTONE
OPINIONS: The Gemara says that if a person builds a gravestone over a grave, the stone becomes Asur b'Hana'ah. Does this imply that sitting or leaning on a gravestone in a graveyard is prohibited?
(a) The TUR (beginning of YD 364) cites RABEINU YESHAYAH who, based on the Gemara here, prohibits one from using a gravestone for any personal purpose. The BEIS YOSEF cites the HAGAHOS ASHIRI in the name of the OR ZARU'A (Moed Katan 3:79) who writes that some prohibit sitting and leaning on a gravestone for this reason.
(b) However, the Tur cites his father, the ROSH, who permits using gravestones today, since they are not part of the grave but merely a marker for the grave. Apparently, in the times of the Gemara the gravestones were part of the grave itself and not merely markers above the grave.
This is also the opinion of the RASHBA (in a Teshuvah cited by the BEDEK HA'BAYIS) and the OR ZARU'A (Hilchos Aveilus 423), who write that today's gravestones are erected simply to give honor to the dead, and they are not Asur b'Hana'ah.
The Rashba there and RABEINU YERUCHAM cited by the BACH give another reason to permit sitting on gravestones. They write that it was customary for people to sit on gravestones. Consequently, when the gravestone was erected, it was considered as though it was done with an explicit condition permitting people to sit or lean on it.
HALACHAH: Both opinions are cited by the REMA (YD 364:1). Accordingly, in a place where it is not the accepted practice to sit on gravestones, there may be reason to be stringent (since the second reason for leniency does not apply).
There is another reason to prohibit sitting on a gravestone. The Gemara in Megilah (29a) prohibits one from letting his animals graze in a graveyard, and from acting light-headedly in a graveyard. The MORDECHAI in Megilah there asks why should one be prohibited to let his animals graze there? The Isur Hana'ah prohibits deriving benefit only from the grave itself, but not from the whole graveyard. Moreover, even the grave itself becomes Asur only when one builds a structure in which he places the Mes. The dirt of the grave itself does not become Asur. The Mordechai answers that the Isur in the Gemara in Moed Katan is not because of the Isur Hana'ah, but because of the disgrace to the dead involved in grazing animals there.
The Acharonim (YAD ELIYAHU, cited by PISCHEI TESHUVAH YD 364:2) write that the concern for disgrace to the dead also prohibits a person from sitting or standing over a gravestone. (Leaning on a gravestone might not be considered a disgrace.)
2) THE DEFINITION OF "HAZMANAH"
QUESTION: Abaye and Rava argue about "Hazmanah Milsa Hi." Abaye maintains that designating an object ("Hazmanah") for a purpose which would make it Asur b'Hana'ah (prohibited to derive personal benefit from it) is effective, and even before the object is actually used for that purpose it is Asur b'Hana'ah. The Halachah follows the ruling of Rava, who maintains that "Hazmanah Lav Milsa Hi" -- designating an object for a certain purpose does not make it prohibited until it has actually been used. However, even according to Rava, Hazmanah does cause the object to become Asur when the object is used for the purpose for which it was designated, while using the object without having designated it for that purpose does not make the object Asur b'Hana'ah (and one may derive personal benefit from the object when it is not being used).
What type of act constitutes Hazmanah (according to Abaye, to make it Asur b'Hana'ah without using it, and according to Rava, to make it Asur b'Hana'ah after it has been used)? From the wording of the Gemara's initial statement ("ha'Oreg Beged l'Mes"), it seems that it is not enough to designate the object for that purpose verbally, but rather some action must be done to make the object fit for use. The Gemara mentions "ha'Oreg Beged," "one who weaves a garment for a dead person," and it does not merely mention a person who "designates" a garment. This is also implied by the Gemara's proof from the case of a person who builds a Nefesh, a gravestone. The Gemara attempts to support the opinion of Abaye from the Beraisa that says that if one adds a new row of stones to the gravestone, it becomes Asur. This implies that simply designating the preexisting stone for the Mes would not prohibit it, according to Abaye.
On the other hand, other indications in the Gemara imply that a verbal declaration designating the object for use does suffice as Hazmanah. The Gemara attempts to bring proof for Abaye's opinion from the Mishnah (Kelim 28:5) which discusses a case of a kerchief ("Kipah") that was designated for use as a cover for a Sefer Torah. Once the Kipah has been designated for use for the Sefer Torah, it becomes Tahor. The word "Nasnaso" implies that it was merely designated, but not changed physically. Also, the Gemara (end of 48a) implies that according to Abaye, if parents throw clothing onto the corpse of their child, the clothing becomes Asur even though no change has been made to the clothing.
What is the correct way to understand what constitutes Hazmanah?
(a) The RAMBAN cited by the RAN rules that it suffices to designate an object verbally, or even in one's mind, for a Mes or for another use of Kedushah. When the Gemara discusses "one who weaves a garment," it is merely giving a common case. The same Halachah applies when a person verbally designates an object for a Mes. In the case of the gravestone as well the Beraisa could have said that the person designated the gravestone for a Mes. However, adding a row will be considered Hazmanah even if he does not verbally designate it to be used exclusively for the Mes.
(b) TOSFOS (48a, DH Nesano la'Sefer) concludes that Hazmanah is accomplished not through verbal designation alone, but through a combination of verbal designation and taking the object in one's hand while one verbally designates it for use. This is why the Gemara says that a person must add another level of stone to the gravestone in order to make a Hazmanah, since he cannot pick up in his hand the stones that are already there. When the Gemara discusses "one who weaves a garment" for a Mes, it does not mean that it must be woven for that purpose. Rather, the Gemara means that even though the garment has been woven, Rava still says it is not Hazmanah.
(c) The RAN explains that it is not enough merely to verbally designate and pick up the object. Rather, one must perform a minimal action to show that he is designating it for that purpose, such as transferring the object to a different person's domain in order to designate it for its purpose. Therefore, if a person simply announces that he wants the garment to be used for a Mes, it will not be Hazmanah unless he either weaves the garment or places it on a Mes. In the case of the clothing that was thrown on the Mes, it is considered an action of transferring the clothing to a different domain the moment it is thrown (even before it lands). In the case of the "Kipah" which was designated to be used as a cover for a Sefer Torah, the Mishnah means not only that it was designated, but that it was given ("Nasnaso") to the Gizbar in charge of the Sifrei Torah for the purpose of being used as a cover for a Sefer Torah.
According to all of these opinions, the same form of Hazmanah should be required according to Rava in order to prohibit the object once it has been designated and used.