Gemara

(R. Shimon): If one does any of the following, he is liable for endangering life... he cuts his nails and casts them into Reshus ha'Rabim.

We are concerned lest a pregnant woman walk over them and miscarry.

Suggestion: This is only if he cut with scissors both his fingernails and toenails, and if he did not cut anything afterwards.

Rejection: This is wrong. We are concerned in all cases;

(Beraisa): One who burns his nails is a Chasid. One who buries them is a Tzadik. One who discards them is a Rasha.

Mo'ed Katan 18a (Rav Shemen bar Aba): On Chol ha'Mo'ed, I saw R. Yochanan cut his nails in the Beis Midrash and discard them.

Question (Beraisa): A Tzadik buries his nails. A Chasid burns them. A Rasha discards them.

Answer: The concern is lest a pregnant woman pass over them and miscarry. Women do not frequent the Beis Midrash.

Question: Sometimes they gather the nails and throw them outside!

Answer: Once they changed, they changed.

Rishonim

Rif and Rosh (Mo'ed Katan 10a and 3:19): One who discards nails is a Rasha, for a pregnant woman might pass over them and miscarry. This is only where they fell. If one gathered them and threw them outside, there is no problem, for once they changed, they changed.

Rashi (Mo'ed Katan 18a DH Kivan): Once the nails changed, i.e. they were moved from where they initially fell, they will not harm anyone.

Prishah (OC 241:4): This permits casting nails in a bathhouse where only men bathe, even though the water takes them to the river. This teaches that if one cut his nails in his house, where women are frequent, and some fell, he should sweep the area or blow to move them from their place, and there is no problem. We do not find any difference between toenails and fingernails regarding this, and there is no reason to distinguish.

Pri Chodosh (YD 116:9): One may cast nails where women do not frequent. Rashi explains that once they were moved from where they initially fell, they do not damage. Presumably this is only if they were gathered in one place with intent to scatter them in the new place. A support is the case in which Shmuel cut his nails and threw them at his brother (Mo'ed Katan 18a). Rashi explained that he later gathered them, for one who discards them is a Rasha. Presumably, initially he gathered them. Why did he need to gather them again after he threw them? Rather, if he gathered them in one place with intent to throw them, the initial place is not called their place, and they can damage afterwards.

Eliyahu Rabah (260:7): Perhaps it is permitted only if he casts them outside the room, for this is Shinuy Makom. The Rif connotes like this.

Birkei Yosef (6): The Gemara supports the Prishah. According to Eliyahu Rabah, it should have said 'once they were taken outside, there is no problem.' Rashi supports this. 'Rashi' on the Rif there says 'once they were moved to another place, they do not damage.'

Tosfos (17a DH Sorfan): A Chasid burns his nails, lest they get uncovered. The Aruch explains that it is harmful to burn something from one's body. Even so, a Chasid does so to ensure that no damage will come to others.

Sefer Chasidim (58): One should not grow long fingernails, lest mud enter underneath. Mud or dough under the nail, even what is even with the skin, is a Chatzitzah. Further, perhaps Chelev or an Isur will enter underneath. A case occurred in which a deceased Tzadik appeared, and his face was yellow. One explained that it because his nails were long, and Chelev entered underneath, and he ate hot food without cleaning the nails. Therefore, it is good to cut the nails every Erev Shabbos.

Kolbo (Sof Siman 87): One who cut his nails begins with the left hand, with the ring finger, and finishes with the index finger. He washes after cutting them, due to the poison, and buries the nails.

Poskim

Shulchan Aruch (OC 260:1): It is a Mitzvah to cut the fingernails on Erev Shabbos.

Source (Beis Yosef DH Kasuv): This is from Kolbo, Chayei Olam and Sefer ha'Yirah of R. Yonah.

Maharshal, cited in Bach (DH Leshon) and Taz (1): One should cut his fingernails every Erev Shabbos. If one cannot bathe and cut his hair on Erev Shabbos, it is best to do so as close to Shabbos as possible. There is a good reason why we do not cut nails on Thursday.

Taz (1): I heard that hair and nails that are cut begin to grow on the third day. Therefore, one should not cause that this will be on Shabbos. Therefore, on Thursday one should not cut also hair.

Eliyahu Rabah (DH v'Hinei): Sefer ha'Gan says that one may not cause the nails to grow on Shabbos. I do not understand the Isur, since it happens by itself. A Mishnah (Ta'anis 4:1) permits shaving on Thursday (even though the hair will begin to grow on Shabbos). Even though it discusses Anshei Mishmar (Kohanim serving in the Mikdash), who are busy on Erev Shabbos, if there were an Isur to do so, we would not permit it just because they are busy on Erev Shabbos. However, perhaps for a Mitzvah (cutting the nails for Kevod Shabbos), we are not concerned (for a minor Isur), like we find regarding starting a sea voyage (within three days before Shabbos) - OC 248:1. I say that since they begin growing again on Shabbos (if he cuts them on Thursday), this is not Kevod Shabbos, and all the more so if he cuts them on Wednesday.

Mishnah Berurah (6): It is no Kevod Shabbos to cut them on Thursday, for they begin to grow again on Shabbos. Some say not to cut toenails and fingernails on the same day, and to cut only on Erev Shabbos or Erev Yom Tov. It is proper to cut toenails on Thursday and fingernails on Friday. If one cut where women go, he should sweep them away. Eliyahu Rabah says that it must be outside the room. One must wash his hands after cutting his nails.

Kaf ha'Chayim (15): If hair and nails begin to grow only on the third day, why does a king cut his hair every day? Nothing grew, since he cut it yesterday! We see that they begin growing immediately. When one cannot cut them on Friday, he cuts them on Thursday.

Eliyahu Rabah (4): One who cut his nails on Erev Shabbos should do so before immersing, like a Nidah. If he reads Shnayim Mikra v'Echad Targum on Erev Shabbos, he should cut his nails first.

Gra (DH v'Im): It is a Mitzvah to cut the nails, like it says in Yevamos (48a. A beautiful woman that a Yisrael took in war must "do her nails." R. Akiva explains that she lets them grow long, for this is repulsive.)

Kaf ha'Chayim (9): The part of the nail even with the skin is Tahor. What extends past the skin must be cut, for Tum'ah nurtures from it.

Rema: One should not cut them in order. He begins with the left hand, with the ring finger, and on the right hand, with the index finger. The Siman for the order is 42531 on the left, and 24135 on the right.

Prishah (241:4, citing Avudraham): One must cut in the order 24135 on the right hand, and 42531 on the left. I found another Siman -- "Kashya b'Lo Terutz", which are the initial letters of "Kemitzah Smol Yemin Etzba b'Azharah Lecha Atah Tikotz Yom Revi'i v'Halah Tzipornayim." I.e. do not cut nails after Wednesday, for from Wednesday pertains to the coming Shabbos, so it honors Shabbos. Why to cut in this order is a question without an answer. Chachamim of France say that if one cuts them in order, this causes poverty, forgetting and burying one's children.

Prishah: The Shulchan Aruch gave a Siman how to cut, lest people cut in order. Also, one should not start from the first finger.

Eliyahu Rabah (5): Sefer ha'Gan says not to cut on Thursday, for then they begin to grow on Shabbos. People conduct like this. In Siman 241, the Bach ruled like Avudraham. Why does he rule here like Sefer ha'Gan? Also the Prishah there cited Avudraham to forbid cutting after Wednesday, due to Kevod Shabbos. This is like Sefer ha'Gan, unlike the Bach. Scrutiny in our text of Avudraham shows that one may not cut (from Wednesday) until Erev Shabbos! The Siman "Kashya b'Lo Tirutz", stands for "... Al Tikotz Yom Revi'i v'Halah." Surely, the greatest Kevod Shabbos is on Erev Shabbos, like regarding bathing.

Taz (2): Maharam was not careful about this (cutting in order), and so it says in Kavanos ha'Ari Zal.

Magen Avraham (1): Even though the Ari Zal mocked people who were particular about this, and Maharam was not careful about this, one should be careful about it l'Chatchilah.

Eliyahu Rabah (6): The Avudraham says that if one cuts them in order, this causes poverty, forgetting and burying one's children. Even though Maharam and the Ari Zal were not concerned for this, who will be lenient about a severe danger?! Shevilei Emunah connotes that there is danger even if he cuts two nails in order.

Eliyahu Rabah (7): The Rema is like the second Perush of the Avudraham. The first Perush says to cut 13524 on the right hand. The Shlah brought only the first Perush, and ignored the Perush that the Rema, Levush and Bach brought. The Siman (Kashya) can work for either Perush. Aleph can represent Agudal (thumb) or Etzba (index finger). Before Adam ha'Rishon sinned, his garment was a fingernail (that covered the body). After he sinned, it was removed, and only the nail remained. Since a woman caused him to sin, she can be punished through this. The Prishah says that one should not cut toenails and fingernails on the same day.

Machazik Berachah (260:4): Several hold like the Magen Avraham, that one should not cut his toenails and fingernails on the same day. I say that there is no concern, for the Ari Zal used to cut both of them on Erev Shabbos.

Birkei Yosef (5): Yesh Nochlin (26, Hagahah) brings this from the Ari Zal. This is wrong, for the Ari Zal was not particular not to cut them in order. All the Acharonim say so. The Magen Avraham and Avudraham say that l'Chatchilah, one should be careful. Surely, if there were any concern, the Ari Zal would have been careful. Since he mocked people who were particular about this, there is no Safek. Here, the custom is that no one is careful about this.

Machazik Berachah (260:7): If it suffices to gather the nails and move them, why must one bury them or burn them? This is even harder for the Aruch, who says that it is Midas Chasidus to burn them, for it harms the person. (Why should one harm himself without need?) According to the Pri Chodosh, we can say that if one cut his nails where women go, he must gather and move them, therefore they could damage in the new place, so he must bury them.

Sha'arei Teshuvah (251:4): Machazik Berachah says that some have the custom to cut something else with the knife after cutting the nails, but one should not be concerned for this.

Aruch ha'Shulchan (6): In Nidah, it says that if one cut something else with the knife after cutting the nails, there is no danger.

Note: This seems unlike our text of the Gemara.

Mishnah Berurah (7): Some do not cut their nails on Rosh Chodesh, for so commanded R. Yehudah ha'Chasid.

Note: Pischei Teshuvah (YD 116:6) brings from the Noda b'Yehudah (2 EH 79) that "Tzavah Zu", i.e. all R. Yehudah ha'Chasid's commands, were only for his descendants, but not for all of Yisrael. Yabi'a Omer (8 YD 20) says that many say so. Va'Ya'an Yosef (OC 313) understands that "Tzavah Zu" refers only to what the Noda b'Yehudah discussed there, i.e. marriages in which the parents have the same name. Divrei Yatziv (EH 12:5) suggests that his commands in matters like this (such marriages) were only for his descendants.

Kaf ha'Chayim (12): If Rosh Chodesh is Erev Shabbos, one should cut them on Thursday. If he did not, and they are long, he may cut them on Rosh Chodesh, for there is a big punishment (for having long nails on Shabbos).

Kaf ha'Chayim (11): Ru'ach Chayim says not to cut them on his garment, for this causes evil to himself, but it seems that the Ari Zal was not concerned for this.

Be'er Moshe (6:133): The Ri of Lunil says that after a day or two, they changed, and they do not harm.