Easy open knives feature a notch near the wide end of a common jack knife pattern, typically a swell end knife.  The spear point blade can either be opened with the nail pull or by grabbing the exposed portion of the blade in the notch.  Sometimes a nail notch is not even present on the blade of an easy open knife.

Camilllus Cut Co Easy Open Knife
Early Camillus Cut Co NY Easy Open Knife

Easy open knives were especially popular from the early 1900s to the 1950s and were made by companies such as Robeson, Remington, Case XX, Camillus, Schrade Cut Co, and more.

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