Gemara

17b (Beraisa - R. Yehudah): Just like shaving is forbidden on Chol ha'Mo'ed and during Aveilus, also cutting nails;

R. Yosi permits.

(Ula): The Halachah is like R. Yosi regarding Chol ha'Mo'ed, and like R. Yehudah regarding Aveilus.

(Shmuel): The Halachah follows R. Yosi regarding both of these.

18a - Suggestion: Only fingernails are permitted!

Rejection: Rav Anan taught that also toenails are permitted.

R. Chiya bar Ashi: One may not use Genustri (scissors).

R. Shemen ben Aba: I saw R. Yochanan cut his nails with his teeth on Chol ha'Mo'ed in the Beis Medrash.

We learn three things from this. One may cut his nails on Chol ha'Mo'ed; it is not considered disgusting (to bite them off in front of others); and it is permitted to cast them on the ground.

Question (Beraisa): One who burns his nails is a Chasid (it is harmful to burn something from one's body. Nevertheless, he does so to ensure that no damage will come to others.) One who buries them is a Tzadik. One who simply drops them is a Rasha.

Answer: Nails are dangerous because a pregnant woman might over them and miscarry. Women are not frequent in the Beis Medrash.

(Rav Yehudah or a Beraisa): Rebbi was asked about cutting nails, and he permitted. Had they asked about cutting the mustache, he would have permitted.

(Shmuel): Rebbi was asked about cutting nails and the mustache, and he permitted both.

(Rav): One may cut the mustache from one end (of the mouth) to the other.

(R. Ami): This is only if it interferes (with eating).

Rav Nachman bar Yitzchak: To me, it is as if the entire mustache interferes (I am an Istnis).

Rishonim

Rif and Rosh (3:19): The Isur to use scissors is for an Avel. During the Mo'ed it is permitted. Some forbid even during the Mo'ed. They bring a proof from R. Yochanan, who cut his nails with his teeth on Chol ha'Mo'ed. I disagree. The Gemara said that we learn three things from that episode. It does not say that we learn a fourth, that one may not cut with scissors! It said that he was in the Beis Medrash so we will know that he used his teeth because scissors were not available, and we will not think that it is forbidden with scissors. One may cut the mustache on Chol ha'Mo'ed.

Note: Seemingly, it needed to say that it was in the Beis Medrash to explain why he was not concerned lest women step on them!

Rebuttal (Ba'al ha'Ma'or): Granted, we cannot learn from the episode that scissors are forbidden. But R. Chiya bar Ashi forbade scissors Stam. It is difficult to say that he refers only to Aveilus!

Defense (Milchamos Hash-m): R. Shemen taught that the episode was in the Beis Medrash so we will not think that scissors are forbidden. I.e., they are permitted. R. Chiya taught about Aveilus. We cannot learn to Chol ha'Mo'ed, for Chol ha'Mo'ed is more lenient. We find that Ula forbids like R. Yehudah regarding Aveilus, but permits like R. Yosi regarding Chol ha'Mo'ed. Shmuel said Rebbi permitted cutting nails and mustache. Presumably these are similar, i.e. both with scissors. In the Yerushalmi, Rav says that nails and mustache are the same in every respect.

Nimukei Yosef (DH Tanu): We allow both fingernails, which are repulsive (if they are too long), and toenails, which are cut merely for comfort. One may cut the mustache, i.e. what enters the mouth and interferes (with eating). If an Istnis is bothered by it, he may cut it regardless of its length.

Rambam (Hilchos Yom Tov 7:20): One may remove his mustache on Chol ha'Mo'ed and cut his nails, even using a Keli.

Magid Mishnah: Some permit to cut only the part of the mustache that interferes with eating, or for an Istnis. The Rambam permits entirely on Chol ha'Mo'ed.

Ra'avad (brought in Rosh 20): Rav permitted cutting the mustache above the mouth from one end to the other (unconditionally). Hanachah, i.e. on the sides, one may cut only what interferes with eating. R. Ami permits cutting above the mouth only what interferes with eating. The Halachah follows R. Ami, for he is Basra.

Rebuttal (Rosh): The Ge'onim rule like Basra from Abaye and Rava and afterwards. Before them, the Halachah follows the Rebbi against his Talmid. Also, we are lenient about mid'Rabanan laws. The Rif permits cutting without distinction.

Tosfos (DH uvi'Gnustri): The Aruch says that Genustri is a special Keli for cutting nails. The Gemara says that Genustri is forbidden. This implies that a blade or scissors is permitted. However, R. Yochanan used his teeth. This suggests that anything else is forbidden! Perhaps he rules like R. Yehudah who forbids cutting (normally) on Chol ha'Mo'ed, unlike Ula and Shmuel. If so, R. Yochanan is more stringent about Chol ha'Mo'ed than about Aveilus, contrary to Rav Shisha brei d'Rav Idi. R. Tam says that cutting nails on each other is a Shinuy regarding Shabbos, and is permitted on Chol ha'Mo'ed.

Poskim

Shulchan Aruch (OC 532:1): One may cut his fingernails and toenails on Chol ha'Mo'ed, even using a scissors.

Beis Yosef (DH u'Mah she'Chosav): Rabbeinu Yehudah forbids with a scissors but permits with a knife, for this is not the normal way. R. Baruch holds that even with a knife is normal, and he forbids. The Rif, Rambam and Rosh permit even with a scissors.

Rema: Some forbid. Our custom is to be stringent not to cut with a knife or scissors, unless needed for the Mitzvah of Tevilah (of women). Then, one may cut them normally, like on a weekday.

Mishnah Berurah (2): The stringent opinion holds that also regarding nails we decree lest one enter the Mo'ed unkempt. Anyone allowed to shave during the Mo'ed may cut his nails. It seems that one who cut his nails on Erev Yom Tov may cut them during the Mo'ed, for some permit shaving in this case. Even though the Halachah does not permit shaving in this case, we may be lenient about cutting nails, for some permit in any case.

Kaf ha'Chayim (4,5): Some Sefardic communities are stringent like the Rema. Some are stringent even for one who cuts his nails every Erev Shabbos. It seems that he may rely on the Mechaber.

Beis Yosef (DH Kasuv): R. Meir of Inglitira writes that if a woman's Tevilah occurs on Chol ha'Mo'ed, she should not cut her nails herself with a scissors. We derive that also a knife is forbidden. Rather, she asks a Nochris to cut them. Even though telling a Nochri is Shevus, we are not stringent when it is needed for a Mitzvah. She herself cannot cut them well with her teeth or hands. If no Nochris is available, she should clean out the dirt underneath them as well as possible, or cut them with a Shinuy, e.g. with her teeth, but not with a scissors or knife. Tosfos (ibid.) permits her to cut them normally, just like one may cut hair of the underarms or Ervah.

Kaf ha'Chayim (8): We permit a Nochris to cut her nails. The Yisraelis' help is insignificant.

Darchei Moshe (1): One may rely on those who permit normally for Tevilah, since some are lenient in any case. If not, there is concern lest she not clean out all the dirt underneath.

Taz (1) and YD (198:21): If her Tevilah was on Yom Tov and she forgot to cut her nails beforehand, she may not ask a Nochris to cut them. We do not permit for the sake of the Mitzvah, because Tevilah can be done without cutting the nails; rather, she cleans well underneath. Also, she is prone to help the Nochris by moving her fingers.

Shulchan Aruch (OC 531:8): Anyone may remove his mustache on Chol ha'Mo'ed.

Kaf ha'Chayim (40): 'Anyone' shows that the Halachah does not follow Rav Nachman bar Yitzchak, who permits only an Istnis.

Beis Yosef (DH veha'Rambam): The Rif and Rambam do not discuss Hanachah. It seems that their text, like ours, omits it. They rule like Rav, who permits cutting the entire mustache. We are lenient, since it is a mid'Rabanan law.

Rebuttal (Gra OC 531:8 DH Kol): The Magid Mishnah connotes that the limitation (to what interferes) applies to Aveilus. The Rif could say so, but not the Rambam, who totally forbids an Avel! The Beis Yosef says that the Rif and Rambam have our text and rule like Rav. This is wrong, for according to our text, R. Ami explains Rav! Also, Rav Nachman bar Yitzchak holds like R. Ami, and Rav Nachman bar Yitzchak is Basra (he was after Rava). Rather, the Rif and Rambam hold like the Aruch. If one cut his mustache but left over part, Rav permits cutting what interferes. R. Ami always permits what interferes. The Rif is lenient like R. Ami because this is mid'Rabanan. The Rambam rules like him because his opinion is brought last.

Beis Yosef (DH Kosav ha'Mordechai): Tosfos says that Rebbi permitted cutting the mustache through a Shinuy, like fingernails. Others permit without a Shinuy. We are lenient, since it is mid'Rabanan.

Magen Avraham (12): Likutei Pardes permits shaving around a bloodletting wound due to mortal danger, but not around a regular wound to put a bandage on it. He must forbid removing the mustache unless it interferes. We permit removing the entire mustache, and all the more so shaving around any wound.

Rema: This is permitted openly.

Gra (DH Afilu): This is because it is clearly for the need of the Mo'ed.