Link (Ex)
Source: Advanced Player's Guide, p. 58
A summoner and his eidolon share a mental link allows for communication across any distance (as long as they are on the same plane). This communication is a free action, allowing the summoner to give orders to his eidolon at any time. In addition, magic items interfere with the summoner’s connection to his eidolon. As a result, the summoner and his eidolon share magic item slots. For example, if the summoner is wearing a ring, his eidolon can wear no more than one ring. In case of a conflict, the items worn by the summoner remain active, and those used by the eidolon become dormant. The eidolon must possess the appropriate appendages to utilize a magic item.