The swell center whittler is a pocket knife that has a “swell” in the middle of the handle, usually coming to a point. Often times, the end bolsters will also come to a point. The three blades are commonly: a primary spear point, a pen, and a coping.
A whittler pocket knife frame is a three blade pen knife with a large blade on one end, and two small blades at the back, at least that’s the official definition. More…the knife has two springs – the two small blades work on one spring each, while the large blade bears on both springs. This is the original definition of a whittler pocket knife, but this conflicts with the actual usage of the term “whittler” by knife manufacturers.
There are some who market a pocket knife as a whittler, even though it has the large blade on the same side as one of the smaller blades. Case is one such manufacturer (in the past). So as with many things knives, the reality is that there is no single definition of a whittler.
Typically, a whittler pocket knife can be determined by its placement of one large primary blade (a spear point in the case of the swell center whittler) on the front, while two smaller blades reside on the back end. And, as pointed out, manufacturers may “adjust” this definition at their own discretion. Often times, the head bolster is slightly larger with the primary anchored to it. Then, the handle narrows toward the back end.