Chulin 18 - Kosher Slaughter - The Place of the Cut
The slaughter knife needs to be absolutely smooth, without notches. It needs be examined using the flesh and the nail of a finger. In addition, the appointed slaughterer needs to show his knife for inspection to the local Sage, as a way of expressing the honor due the Sage.
The correct place for the slaughter cut is the front of the neck of the animal, and the knife should cut more than half of the trachea and esophagus. The trachea itself is encircled by many successive rings of cartilage, which are horseshoe shaped. The uppermost ring is a complete ring, and it forms the upper limit of the cut. If he cut above the upper ring, the shechitah is not kosher. Rabbi Yose ben Yehudah says that as long as he cut the major part of the trachea within the upper ring, it is valid, even if the knife went higher in the remaining part of the cut. Rabbi Chanina ben Antignos testified that even if he cut above the uppermost ring, but still under the thyroid, it is valid, and his is the accepted point of view.
Art: Leon Bonvin - Still Life With Basket Of Grapes, Walnuts And Knife