Reference for Chullin 34:28

והאמר רבי יוחנן

R'Papa ruled: It must be examined with the flesh of the finger and with the fingernail, and the examination must be of the three edges [of the knife].<span class="x" onmousemove="('comment',' I.e., the sharp edge, and also the sides of this edge must be examined.');">¹⁷</span>

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