Siman 8

A knife for shechitah has no fixed measure for its length. Thus our sages, of blessed memory, said [31a] that if one slaughters with a forward motion alone or a backward motion alone, the knife must be as long as the measure of the neck of the item being slaughtered, including the skin and the vertebrae, and beyond the neck the full measure of a neck; that is to say, its length should be like two necks with the skin and the vertebrae. For when the knife has such a measure, it is sufficient to slaughter without derasa in a forward motion or a backward motion. If it does not have this measure, the simanim are not slaughtered without derasa [Ran]. And when it has this measure, it is not necessary that he move the entire length of the knife before he slaughters the majority, for a person cannot be careful in this, as sometimes when the knife is sharp and keen, it cuts the majority of the simanim before he moves it forward or backward this measure. The sages stated this measure because they established that then he will slaughter easily without derasa [the Rosh]. It will be further explained in siman 24 with the help of Heaven.

However, when one slaughters with a forward and backward motion, which is the essence of slaughtering ab initio as written there, no measure is required at all, for through a forward and backward motion one will not press even with a small knife. And so the sages taught in the Mishnah [30b]: but if he moved it forward and backward even a minute amount, even with a lancet, it is kosher. A lancet is a very thin and small razor. There were those who were accustomed to make horns for the lancet for beauty, which lean toward the first side, and with these it is forbidden to slaughter, for since it is very small it slips from the neck, and when he moves it forward and backward there is a concern lest the simanim be hidden or those horns will puncture them [Rashi]. It is permitted to slaughter with those that do not have horns, and we do not decree on account of those that do have horns [Gemara, and this is the novelty in the Mishnah, and examine it closely]. And specifically a lancet that has at least some length, for those that have no length at all are like a needle that punctures and tears and does not cut. Even a shoemaker's needle, which is somewhat thick and its edges are sharp such that they cut the sewing threads, the slaughtering is also forbidden, for it punctures the simanim and does not cut. And that which we learned in the Mishnah "even a minute amount," nevertheless it has some measure [as written by the Tosafot, Shabbat 63b, s.v. Arig]. From the words of the Rambam, the Tur, and the Shulchan Aruch, it appears that they interpret "horns" as a point, meaning that it is not wide or round at the head of the lancet but a sharp point, for there is a concern that due to its smallness he will stab with the point; for with a small lancet, when it has a point, the slaughtering is forbidden. And we are stringent even with a large knife that it should not have a point, but this is merely a stringency, whereas with a small lancet it is by the letter of the law [see Turei Zahav, subsection 1].

And there are some of our early teachers [the Rif and Rabbeinu Chananel] who wrote that this which is valid even with a knife of any small size through a forward and backward stroke is only regarding a bird and not regarding an animal, for the neck is a proper length, and when the knife is very small, it is impossible to slaughter even with a forward and backward stroke unless it is close to pressing. Therefore, they wrote that regarding an animal we require the full width of the neck in any case, and also a bit beyond the neck. And this is the difference between a forward stroke alone or a backward stroke alone and between a forward and backward stroke: that we do not require beyond the neck the full width of the neck [and according to their words, "any size" taught in the Mishnah refers to beyond the neck, see there]. And some say further that even regarding a bird we require the full width of the neck and any size beyond the neck [and as I wrote, the Mishnah refers to beyond the neck]. And although there are those who disagreed with them, nevertheless many of the great authorities seized their words as the law [see the Shulchan Aruch there and the Bach]. And it is a simple matter that in practice one should not slaughter even a small bird unless with some proper length and not with a very small knife; and conversely, one should not slaughter a bird with a knife for large animals, as it is heavy and close to coming to pressing, unless in a time of pressing need and with extra caution [and so wrote the Tevuot Shor].

Our teacher, the Rema, wrote that one who is unable to estimate should take a knife length equal to two necks of the animal being slaughtered, and some say to measure fourteen fingers. There is an allusion to this in that which is stated, "and slaughter with this [be-zeh]" (I Samuel 14), the numerical value of "zeh" being fourteen. Thus far his words. This allusion is found in the Midrash, Parshat Naso (Chapter 10), and this is its wording: What is "and slaughter with this"? The Rabbis say: He showed them a knife, how much its length must be; from where is this derived? From "be-zeh": "bet" is two, "zayin" is seven, "heh" is five, etc. See there. It is merely a general allusion, and even according to this allusion, it only applies to large livestock, as it is written there, "And all the people brought each man his ox... and slaughtered them there," and not to small livestock or birds. Experience testifies that it is difficult to slaughter birds with a knife as long as this. [And previously it is written there, "Bring to me each man his ox and each man his sheep, and slaughter with this."]