A Beraisa lists the various types of rip that may never be resewn:

A rip made over a parent

Over one's rabbi, a Nasi, or an Av Beis Din

Over hearing bad news

Over hearing blasphemy

Over seeing a Sefer Torah destroyed

Over seeing the cities of Judah, the Temple, and Jerusalem in destruction.

The Beraisa describes the procedure for Keri'ah over the above mentioned holy places: First one does Keri'ah when seeing the Temple in destruction, and then adds to the rip when he sees Jerusalem.

The Gemara brings proofs for the above cases - that they require Keri'ah and that the rip may not be sewn.

The Gemara notes that case 1:a:3 (bad news) only applies when the bad news is about a majority of the Jewish people.

Concerning 1:a:4 the Gemara brings a Beraisa that states: The one who hears the blasphemy (hearer #1) does Keri'ah, and so does someone who hears the report from the one who heard (hearer #2). But when hearer #1 repeats the blasphemy before hearer #2 he does not have to do Keri'ah again. This is derived by the Gemara from a Pasuk.

The tear may not be mended.

The Gemara discusses the story of Yehoyakim burning the scroll (namely, Eichah) written by Yirmiyahu, described in Yirmiyahu 36.

Two details concerning 1:a:5 (Sefer Torah destroyed):

R. Huna said that two Keri'os are required - one for the Sefer Torah per se and one for its content.

Keri'ah is only necessary if one could have stopped the destruction of the Sefer Torah but was prevented from doing so. But if it happens suddenly, by an accident (e.g., it is snatched by an animal), Keri'ah is not done.

Concerning 1:a:6 (the cities of Judah, etc.), the Gemara brings Pesukim that should be said while doing Keri'ah for these three holy places.

The Gemara discusses 1:a:7:

Question: Another Beraisa states that two separate rips are made for Jerusalem and for the Temple. This contradicts the above Beraisa that says that the rip is made for the Temple and then the same rip is widened for Jerusalem.

Answer: The first Beraisa (1:a:7) is talking about someone who encounters the Temple Mount before seeing Jerusalem. The second Beraisa is talking about someone who saw Jerusalem first and then the Temple Mount.

(Beraisa): All these rips (1:a:1-6) may be sewn up; folded together and loosely stitched (Rashi MS.); gathered together and loosely stitched without folding one side over the other (ibid.); or loosely stitched without joining the two pieces close together. The only thing you can't do is sew them properly.

R. Chisda explains what "sewing properly" means - Alexandrian stitching.

Another Beraisa states that if a Keri'ah is done not on the material itself, but along one of the aforementioned types of stitching it is not valid. Along a proper stitch it is valid.

Once again R. Chisda explained that "a proper stitch" means an Alexandrian stitch.

The Tana Kama says that if the garment is altered so that it is to be worn upside down, the stitch (which is now towards the bottom of the garment) may be resewn. R. Shimon ben Elazar, however, forbids this.

The Beraisa also says that even if a second person buys the garment with the rip (that was done for one of the reasons mentioned above in 1:a:1-6) he may also not sew it.

A Beraisa lists three opinions:

(R. Meir): The first Keri'ah must be a tefach. If a new rip has to be added to the first one (over an additional calamity) it need be only three fingerbreadths.

(R. Yehudah): The first Keri'ah must be three fingerbreadths, and the addition can be any slight amount.

(R. Yosi): The first Keri'ah must be a tefach, but the addition need be only a slight amount. (Ula ruled like this.)

A Beraisa is quoted that makes several points:

If a Keri'ah was made originally for a parent and then extended for another relative, the top (first) part may not be sewn and the other (second), additional part may be. If vice versa (the top Keri'ah was for another relative and the bottom for the parent) the bottom part may not be sewn but the top may.

If someone heard about the death of both parents and other relatives at one time there is a dispute. The Tana Kama says one Keri'ah covers all, and R. Yehudah Ben Beseira that the two parents' Keri'ah can be covered by one rip, but the others cannot be mixed in with the parents' rip.

Shmuel ruled like R. Yehudah Ben Beseira. Although he is not the most lenient opinion, the rule about following the lenient opinion in matters of mourning does not apply to the laws of Keri'ah.

The Gemara brings a Beraisa, which discusses two points:

How far may a multiple rip extend? One opinion says until the navel, another says until the heart. After that point a new rip must be made, in a different part of the garment.

Keri'ah done from the bottom of the garment is not valid. The Kohen Gadol, however, must do Keri'ah from the bottom.

There is a dispute as to when an existing rip may be extended:

One opinion says that if the second cause for Keri'ah arises during Shiv'ah a new rip must be made, but during Shloshim the old one may be extended. The other opinion says that during Shloshim a new rip must be made; only after Shloshim may the old one be extended.

The reason the first opinion requires a new rip during Shiv'ah is because during that time the (first) rip may not be sewn up (above, 22b, 2:i); it is only after the rip has been sewn up that a further Keri'ah on that spot is noticeable.

Question: If so, in the case of a woman, who may mend her rip immediately (above, ibid.), she should be able to extend the rip even during Shiv'ah.

Answer: The mending in that case is not a halachic mending, but is only for the sake of modesty. This kind of mending is not sufficient for doing Keri'ah on it.

The reason the second opinion requires a new rip during Shiv'ah is because during that time the (first) rip may not be resewn (ibid.); it is only after the rip has been resewn that a further Keri'ah on that spot is noticeable.

Question: In the case of a parent, where the rip may never be resewn, it should be forbidden to extend a Keri'ah forever.

Answer: The prohibition to resew in that case is not a halachic prohibition, but is only a gesture of honor of the parent. This kind of prohibition is not sufficient for preventing an extension of an existing rip.

A Beraisa is brought that makes two points:

If someone puts on an already-ripped garment for a dead relative (instead of ripping one anew), it is tantamount to stealing from the deceased (by depriving him of his due) and from the living (by deceiving everyone into thinking that he did Keri'ah).

(Raban Shimon Ben Gamliel): If one is wearing a borrowed garment when he hears of a death he should not rip it, unless he hinted to the lender that he might have to do Keri'ah on it. In that case he can rip it and then resew it, return it, and pay for the damage.

Another Beraisa is brought that makes several points:

A sick person should not be informed of a relative's death (nor should anyone be allowed to mourn in his presence) if the news may complicate his state of health.

A child mourner's clothing should be ripped (even though he is too young to participate in mourning), to move other people (who see it) to grief.

One should do Keri'ah for a parent-in-law, out of respect for his wife.

Another Beraisa is brought that says that a mourner should not play with a baby, because it makes him happy and playful, which is inappropriate.

(Beraisa): When we say that during the mourner's meal the consolers (and mourners too, out of regard for the consolers - Ritva) should sit on overturned beds, this is only if the consoler is very close with the mourner. If he is not very close he (and the mourner) sit on upright beds.

Story: Rava became a mourner, and someone came to bring him the mourner's meal. Rava sat on an upright bed (because he did not consider the consoler to be so close with him) but the consoler (inappropriately) sat on an overturned bed.