Furniture, when placed in your Mog Safe, can be laid out in your Mog House to make it look less sparse. Additionally, your moogle can use your furniture as extra Storage space, allowing you to fit more things in your Mog House.
Every piece of furniture has a storage value, and an elemental affinity. The more of one element you have in your Mog House, the greater the elemental aura your house emits. Opposing elements, if arranged in your Mog House at the same time, will cancel each other's auras out. Like storage value, a furniture's elemental affinity can be more than one, but unlike storage, you are not told how strong a piece of furniture is tied to its element. All pieces of furniture have a storage value of at least one.
After placing certain furniture in your mog house and meeting certain conditions, your moogle may interract with you about a specific piece in some way. For example, you may be given a quest (such as the quests to expand your mog house), or your moogle may "find" an item of some sort and give it to you (such as an Ether). Details regarding which furniture pieces can trigger these sorts of reactions from your moogle are listed here: Furniture Quest.
Finally, furniture will give your moogle special powers, based on the type of furniture you have in your Mog House, and how strong the elemental aura in your house is. These powers are called Moghancements.
Adding Furnishings to the second floor
To add your furnishings to the second floor, you must be on the second floor. Once you're on the second floor, open the side menu(Square for PS2, X for Xbox 360 or the "-" on your keyboard sidebar). Instead of "Maps" you will see "Mog House" at the bottom. That opens the Moogle menu and the rest is the same as first floor. You can put your furnishings in either Mog Safe or Mog Safe II. You will have access to both to add your furnishings.
Furniture attributes by name
- Furnishings Guide: A comprehensive furniture list, including merchants that sell furniture and craft skill caps.