Knight of the sword
First of the knight classes, with great skill at war and grasping a sword, it's considered the best servant class and they generally possess the highest stats.
Example heroes: Arturia Pendragon, Miyamoto Musashi
To force the servant you are summoning into the class of Saber you just add these three lines to the end of your summoning chant.
"Thy sword rests upon the throne of knights.
Thou art thee who serves no other.
I am he who holds thy sheath. "
As listed on the Servant category page.
Their free Class skills are:
Eternal Arms Mastership D
Sabers are then allowed to pick up to five skills for up to 37 points.