To clarify a bit, hopefully without fanning any flames: When Spyderco first started building knives, they applied for and were granted a patent for a depression in the blade which allowed purchase for the thumb to facilitate one hand opening. The particular application of the general patent that they settled on was a round hole, although they did use a trapezoidal depression on one model. While the patent was still in force, Spyderco applied to have one particular incarnation of the depression, namely the round hole, registered as a trademark of their company. That application was approved, and the round opening hole is in fact a registered trademark of Spyderco. The patent has run out, so anyone can use holes or depressions without infringing on a patent. They can use holes of shapes other than the round hole (or now the comet hole of the Byrd line which has also been registered as a trademark) freely.
This is where I stop to avoid fanning flames.