Gemara

(Rav Yehudah): If the launderer did not remove three threads (to even out a garment), they count towards the amount that the Techeiles strings (of Tzitzis) must be distanced from the corner.

Menachos 41b (Rav Huna): The threads must be within four (Etzba'os from the corner).

(Rav Yehudah and Rav Papa): They must be within three.

42a (Beraisa): If one attached threads on the Keren (corner), or on the Gedil (a strong thread on the border, or threads hanging from the edge of a garment), it is Kosher;

R. Eliezer ben Yakov says, both of these are Pasul.

(Rav): Tzitzis must hang over the Keren.

He holds like R. Eliezer ben Yakov.

(R. Yakov): One must distance the Tzitzis (from the Keren) a Godel.

We need the teachings of R. Yakov and Rav Papa. Rav Papa gave the maximum distance from the Keren. One might have thought that it can be as close as desired, and the closer the better! R. Yakov gave the minimum distance from the Keren. One might have thought that it can be as far as desired, and the further the better!

The corner of Ravina's garment tore, and the Tzitzis was no longer a Godel from the Keren.

Rav Sama (to Ravina): Do you argue with R. Yakov?!

Ravina: That applies only when attaching the Tzitzis.

Rishonim

The Rif and Rosh (Hilchos Tzitzis, after Menachos 12b and 14) bring the Gemara in Menachos.

Rambam (Hilchos Tzitzis 6:1): To make Tzitzis, one begins from the Zavis (corner) at the end of the weaving and and distances between Kesher Godel (a thumb joint) and three Etzba'os.

Mordechai (948): One inserts the strings within three Etzba'os of the corner. Further than this is not called 'corner.' They must be at least Malei Godel from the corner, for it says "Al the corner", not 'below the corner.' It is good to make a border on the edge, for sometimes threads separate from the garment, and it is a Safek from where to count the three Etzba'os.

Poskim

Shulchan Aruch (OC 11:9): One makes a hole in the length of the Talis, not more than three Etzba'os (Rema - i.e. Godelim), for is not called 'corner.' It may not be less than the distance between the joint of the thumb until the nail, for it says "Al the corner." If it was less than Malei Kesher Godel, it would be below the corner.

Beis Yosef (DH v'Ya'aseh): The hole must be (far from the end) at least the distance from the joint of the thumb until the end of the nail. 'Keren' is the end of the garment, i.e. the bottom. Because Tzitzis must be near the bottom, where the corner is, it is called Keren. Some say that the Tur refers to the corner, and measures three Etzba'os from it on the diagonal. This is wrong. If so, why did he say 'in the length of the Talis'? Also, he and Semag say that there is no Shi'ur in the width of the garment. Also the Rambam agrees. If he held that one measures along the diagonal, he would have said so explicitly. He said that one begins from the Zavis, because one must put near the Zavis, where the corner is. One measures the straight distance (from the bottom); everyone does so. If one measured (Kesher Godel) on the diagonal it is Pasul, for there is less than Kesher Godel from the bottom, since for each unit measure of length, the diagonal is (bigger, i.e.) one and two fifths.

Beis Yosef (DH v'Makom): If he put it exactly three Etzba'os away, it is Kosher. Only further is Pasul. The Roke'ach forbids putting it three Etzba'os away. We must strain (to explain that he means more than three) to avoid saying that he argues with Rashi, the Rambam and Semag.

Beis Yosef (DH v'Chosav): The Kolbo, Semak, Roke'ach, and Mordechai say that 'Etzba' is a Godel. The Rambam says that it is a thumb width. Many people measure the three middle fingers. They err. They have no one to rely on. The Mordechai says that we measure from where the thumb is wide. Tosfos (Menachos 41b DH Arba'ah) was unsure about this.

Beis Yosef (DH v'grs): Semag says that within Malei Kesher Godel is not "Al the corner", rather, below the corner. One may not put Tzitzis in the border, for it is not called 'garment.' Rashi holds that the last Kesher Godel is called Keren. Between one and three Godelim is called Kenaf (corner). Above three is the garment itself.

Beis Yosef (DH uv'Kolbo): The Kolbo says that the hole must be at the end of three Etzba'os. This is difficult, for the Gemara says that past Kesher Godel suffices!

Question (Beis Yosef DH v'Rabbeinu): Mahari Avuhav says that it must be between at most three because less than three is not called the garment. It must be more than one, for less than this is not called Kenaf. I do not understand this. Less than three is Kosher because the Torah does not require on the garment, rather, on the Kenaf! According to Mahari Avuhav, within three should be Pasul, but the Gemara says that it is Kosher!

Answer (Taz 9): Mahari Avuhav says that since within three is not called Beged, the hole must be within three, where it is called Kenaf!

Taz (8): Godel is where the thumb is widest, not at the end where it is narrow.

Bi'ur Halachah (DH Hainu): This is the primary opinion. However, one must make the hole within three Etzba'os. It is best to make the hole within the width of the pinky, ring and middle fingers, for this also fulfills the opinion that we measure where the thunb is thin.

Kaf ha'Chayim (27): Only the Mordechai and Rambam definitely say to measure from where it is widest. The Ari Zal and many say that we measure from where it is narrow.

Beis Yosef (Sof ha'Siman DH Kosav): The Sifri explains that on the Keren is Pasul because then it has eight, and the Torah said four. Ba'al ha'Itur explains that the way we put Tzitzis on, we can use them also if the garment is turned inside out, so it is as if it has Tzitzis on eight corners. He disqualified all our Tzitzis, and said that his Rebbin a Yishuvm make two holes in the garment and insert the Tzitzis through them, so all are on one side of the garment. To defend our custom, he explains the Medrash to disqualify Tzitzis diagonally on the corner. It is as if each Tzitzis is on both sides, therefore it is like eight. The Yerushalmi says that whenever the Halachah wavers, we follow what people do. One who is stringent about such matters is not stringent, rather, he is shocking, for it looks like haughtiness.

Bach (DH v'Ika): Surely, the Sifri disqualifies only when one actually affixed eight Tzitziyos. Ba'al ha'Itur means that l'Chatchilah, one should make two holes, lest it seem like it has eight. He did not disqualify b'Di'eved! However, Mahari Avuhav (cited in the Beis Yosef) understood that he disqualifies b'Di'eved. It is proper to be stringent like Ba'al ha'Itur regarding the Talis Katan, which is under one's garment and others do not see it, so it does not look like haughtiness. Maharil says that one should not hang the Tzitzis on the bottom, rather, on the side.

Magen Avraham (13): Also the Ari Zal did like Ba'al ha'Itur. This is difficult, for the Gemara said that the first knot must be mid'Oraisa, for if not, the Torah did not need to permit Shatnez in Tzitzis, for we hold that one insertion (of a thread through a garment) is not considered attached. According to Ba'al ha'Itur, two insertions are required!

Mishnah Berurah (39): Birkei Yosef says that R. Chaim Vital did not do like the Ba'al ha'Itur. Each locale has its own practice.

Kaf ha'Chayim (24): Surely, R. Chaim Vital understood that the Ari Zal held this is not Me'akev. He merely continued the Ashkenazi custom of his youth. Therefore, R. Chaim Vital did not change his custom. Also Chasidei Beis Kel in Yerushalayim do not do so even on the Talis Katan.

Mishnah Berurah (41): If one inserted the strings more than three Etzba'os from the end, and through tying them the garment shriveled and now there are less than three, it is Pasul, for there is a Shi'ur Beged in between. If one inserted them more than a Kesher Godel from the end, and through tying them it became less than Kesher Godel, it is Kosher, for there is a Shi'ur of Kenaf in between.

Rema: We measure straight, not on a diagonal from the corner.