Quarterstaff

A quarterstaff is a simple piece of wood, about 5 – 7 feet in length.

A quarterstaff  is a double weapon. You can fight with it as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting  with two weapons, just as if you were using a one-handed weapon and a light weapon. A creature wielding a quarterstaff in one hand can’t use it as a double weapon – only one end of the weapon can be used in any given round.

The quarterstaff is a special monk weapon. This designation gives a monk wielding a quarterstaff special options

Weapon Feature(s): double, monk

Two-Handed  Melee Weapons (Simple)
Weapon	Cost	Dmg (S)	Dmg (M)	Critical	Range Increment	Weight	Type	Special
Quarterstaff	–	1d4/1d4	1d6/1d6	×2	–	4 lb	Bludgeoning	double, monk

Source: Core Rulebook