All fighter types are allowed to be specialized in weapon classes.
(Fighter, Paladin, Barbarian, Fighterment, Thief)

You are allowed 3 specializations per character.
(2 for fighterment, 1 for thief)

You may choose to specialize all in one weapon class or up to 3 different ones at one spec level each.

It's recommended to spec all in the same weapon.

The first spec level comes at level 15, Then 17 and lastly at 19.
However, if you want to spec in the same weapon, you spec at 15, 19 and 24.

Each specialization will increase both the effective skill level and combat penetration power of the chosen weapons skill.

Additional bonuses are provided in defeating weapons blocks

Fighters specialized in a weapon receive a skill gain bonus for each level of
specialization they have achieved.

To specialize, visit your trainer and type "NAME, specialize" while holding the weapon you want to specialize in within your right hand.