I believe you are referring to the Guillotine. Used in the middle ages for execution by decapitation. Most famously used on Miss Marie Antoinette of France. Although a gruesome way to die, because of the incredible sharpness and weight of the blade, you would die without feeling a thing.
To make it easier to slice into the persons neck, it also makes it easier to sharpen the blade.
Because they are curved or they are slanted
parallelegram is the name it is a rectangle that is sort of slanted
Guillotines are slanted to allow gravity to assist in the swift and efficient severing of the head during an execution. The angled blade increases the speed and force of the falling blade, ensuring a quicker and cleaner cut. This design feature helps minimize suffering and ensures a more effective execution process.
oblique
A rhombus or a parallelogram
The name of the blade that stabs Frodo is called a Morgul-blade
Guillotine blades are slanted to increase the efficiency of the cutting process. The slant helps the blade slice through the object being cut more smoothly and quickly, resulting in a cleaner and more effective cut.
Another name for a turbine blade is a vane.
The scientific name for the shoulder blade is scapula.
Blade Thompson's birth name is Andreas Kube.
Barrett Blade's birth name is Russell A Barrett.