Short blade
The Short Blade (first available at Rank 3) is a short, curved blade that is a smaller, more agile weapon than the longsword. The Animus, in its hints and tips given to the player, recommends the Short Blade for killing off large numbers of enemies.
Like the Longsword, Altaïr's Short Blade increases in quality and power with his rank.
While the Short Blade is selected, the player can also use throwing knives by holding down the modifier key and pressing the weapon hand key (
on Playstation 3, X on Xbox 360, Left Mouse Button for PC default layout).
Assassin's Creed II

Ezio can carry a short blade in addition to his longsword. Various short blades can be purchased at blacksmith shops with types ranging from traditional daggers, stiletto daggers and pugios. Unlike Altaïr, he won't carry it on his back but next to his longsword.