Reference for Chullin 151:28

ואלו הן צומת הגידין

A certain Rabbi was sitting before R'Abba and recited: It is that part which is on the heel; whereupon R'Abba said: Pay no attention to him, for thus said Rab Judah: It is that part which the butchers strike;<span class="x" onmousemove="('comment',' When commencing to flay the animal, or when about to porge the meat; it is immediately above the tuber calcis.');">¹⁸</span>

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