The Yggdra Union sub series utilizes a weapon type system commonly referred to as Weapon Affinity; it shares some similarities to the "weapon triangle" used in the veteran tactical RPG series: Fire Emblem. There are eight (8) different types of weapons in YU: Sword, Spear, Axe, Bow, Rod, Lump, Scythe, and Book. Below is a chart that shows each weapon's effectiveness against every other weapon affinity.
Blaze Union introduced the Koto weapon, which is unique to the character Eater and neutral against all weapons, in addition to having no disadvantages. This brings the weapon count up to 9.
Note that the Bow weapon affinity was replaced by the identically functioning Gun, and Book was removed altogether.