About forty years ago, a prominent chassidishe rebbe passed away in America. Years earlier he had purchased three cemetery plots on Har Hamenuchos - one for himself, one for his wife, and an extra one. After his death, two of his sons were arguing over which of them would have the privilege to be buried next to their father. The oldest son was in business but felt that since he was the bechor, he should be entitled to the third plot. The younger son took over his father's position as the chassidishe rebbe and he felt that because he was his father's mimaleh mokom in the chassidus, he should be entitled to the privilege of burying buried in the third plot. When they finally agreed upon whom they would present the question to, the rov whom they asked paskened that kol ha'kodem zoche. Why should this be the psak?
In Parshas Pinchas, the Torah speaks about yerusha. The monetary assets of an individual are passed on b'yerusha to his closest relative, and only relatives from the father's side of the family are referred to as "mishpacha". The Minchas Chinuch points out, however, that the Gemara speaks of another concept called "kom tachtov" which is not identical with yerusha. When a married man dies leaving children, the surviving almana does not require chalitza because the children are kom tachtov of the father. An eved kena'ani is considered a monetary asset of his owner, and when the owner passes away, ownership of the eved transfers via yerusha to the closest relative in the mishpacha. An eved Ivri, however, is not considered a monetary asset and therefore should not lend itself to the laws of yerusha, and yet the son does in fact take the place of the father as master of the eved Ivri. This is based on the concept of kom tachtov, and only applies to the master's son and not to his daughter. There is a view in the Yerushalmi that an ama ha'ivriya, upon the death of her owner, is transferred only to the master's daughter and not to his son. These are all details within the concept of "kom tachtov".
The Mishna in Nazir records the following halacha which R' Yochanon explains is a halacha l'Moshe miSinai: if a father and son were each a nazir, and the father set aside animals for the korbanos he must bring upon the completion of his term of nezirus but then dies before he had a chance to bring those korbanos, his son is permitted to bring those animals for his own korbanos at the termination of his period of nezirus. This is a surprising ruling; usually, korbanos have to be designated at the time of their sanctification for a specific purpose, in this case they were designated for the father's nezirus, and yet we allow the son to bring them later for his own nezirus! This halacha is also based on the concept of "kom tachtov". The Gemara in Nazir discusses a slightly more complicated case than the one in the Mishna: what if the father who was a nazir leaves two sons who are both nezirim - do they divide the korbanos designated by the father equally between the two of them, or do we say that kol ha'kodem zocheh, i.e. whichever son's nezirus ends earlier has the right to use all of the father's korbanos for his nezirus? Apparently in Europe it was an accepted practice that the rule of kol ha'kodem zocheh would be applied in such cases. The psak issued by the rov in the case we described earlier (where two brothers who both wanted to be buried next to their father in the last available plot) was apparently based on these ideas that appear in the Gemara.
The Mishna teaches us that if a woman who is not currently married dies, her children inherit her monetary assets, but if children predecease their mother, their mother does not inherit their assets. The reason for this difference is that the relationship children have with their mother is one of "she'er basar" but is not one of "mishpacha". Only relatives from the father's side have the halachic status of "mishpacha", and yerusha only occurs when there is a relationship of mishpacha. As such, a mother does not inherit her children. Children do inherit their mother only because of the idea of "kom tachtov" - children are kom tachas their mother, but a mother is not kom tachas her children.
The laws of yerusha are very clearly spelled out in the Gemara and in the Shulchan Aruch, but unfortunately are not observed properly. There were Jewish communities in the Middle East where they assumed that even Jews should follow dina de'malchusa ("the law of the land") with respect to yerusha. In the sixteenth century, the rabbonim in Tzat sent a young talmid chacham (Rabbi Yom Tov Tzahalon) to explain to these communities that whenever all the parties involved in a monetary issue are Jewish, we follow the Torah law as opposed to dina de'malchusa. The Rambam records a very interesting concept: we consider yerusha to be a matter of issur v'heter, based on the terminology used in the possuk in Parshas Pinchas which describes yerusha as a "chukas mishpat". As such, we should certainly not follow dina de'malchusa regarding yerusha, since dina de'malchusa only applies in areas of dinei mamanos (monetary matters) and not at all in areas of issur v'heter.