See List of Equipment for all equipable items.
There are 16 slots for equipment. Some items take up multiple slots, e.g. the Bear Cape that takes up both the headwear and cape slots, and the Pickaxe that takes up both hands. Equipment slots are unfortunately not labelled or indicated by any sort of picture in-game, making it confusing for new players to know what each slot is for. Players thus label the equipment slots using a code that combines which side of the equipment screen the slot is on and how far it is from the top, e.g. 5R is the fifth slot from the top on the right side.
Slots on the Equipment ScreenEdit
|4L||Left Hand||4R||Right Hand|
While the quiver appears to take up two slots, this is simply a graphical bug and it only takes up the Back (6R) slot.