Yoreh Deah Siman 21 Seif 1

Siman 21 The amount of slaughter- (how far must one cut)

א כַּמָּה הוּא שִׁעוּר הַשְּׁחִיטָה שֶׁל הַקָּנֶה וְהַוֵּשֶׁט; הַשְּׁחִיטָה הַמְּעֻלָּה, שֶׁיֵּחָתְּכוּ שְׁנֵיהֶם בֵּין בַּבְּהֵמָה וּבֵין בָּעוֹף, וְלָזֶה יִתְכַּוֵּן הַשּׁוֹחֵט; וְאִם שָׁחַט רֹב אֶחָד מֵהֶם בָּעוֹף, וְרֹב שְׁנַיִם בַּבְּהֵמָה וּבַחַיָּה, שְׁחִיטָתוֹ כְּשֵׁרָה, וּבִלְבַד כְּשֶׁיִּמְדְּדוּ אוֹתוֹ יִמְצְאוּ שֶׁהַנִּשְׁחָט הוּא רֹב; וְכֵיוָן שֶׁיִּמְצְאוּ שֶׁהַנִּשְׁחָט יוֹתֵר מֵחֵצִי, אֲפִלּוּ כְּחוּט הַשַּׂעֲרָה, דַּיּוֹ.

Seif 1

How much must one cut of the trachea and esophagus. The best way to slaughter is to cut both of them an animal and a fowl side by side this is what he (the slaughterer) should do..

If you cut one sign by a fowl, and one sign of an animal and a beast, the slaughter is acceptable, As long as when you measure it you find that it was slaughtered to the majority, And if you find that the part that was slaughtered was more than half even a hair breadths more than half its enough.