In Path of Exile, many items contain colored gem sockets. Skill gems of the matching color may be placed into these sockets to immediately gain access to that skill. Gems can be removed from sockets at any time with no penalty. The size and type of item will influence how many sockets it can hold. Most armor pieces can have up to four sockets. High end body armor can reach six sockets. One-hand weapons and shields cap out at three sockets, while two-hand weapons can contain six. In addition, the attribute alignment of the item will influence which colors the sockets will be. For example, a Strength/Dexterity sword is very likely to contain red or green sockets and unlikely to have blue sockets.
Sockets can also be linked to adjacent sockets forming groups of linked sockets. A support gem may be placed in a socket linked to a skill gem and the support gem will modify the effect of the skill gem. For example, an item may be found with two sockets linked to each other. A player can place the Fireball skill gem into one socket and the Multiple Projectiles support gem into the other which will allow the player to fire multiple fireballs at once. Placing two skill gems into linked sockets will have no effect beyond giving the player access to both skills. An item can have multiple socket groups.
In the event of three or more linked sockets, as in the item pictured, all of the support gems in the socket group will modify the effects of all skill gems in the socket group. For example, the player may place two skill gems and two support gems into the four linked sockets and both support gems would augment the effects of both skill gems.
The effects of similar skill and support gems do not stack. If two or more of the same gem are equipped, the one with the higher level is used for calculating the effect. However, if two of the same skill gems are placed into two separate socket groups, the skill will benefit from the effects of all support gems linked to both skill gems.