The edges ("edges") attribute is a possibly empty ordered list of edges associated with an atom. As noted in the previous section, an edge may represent either a bond or a gap (Table ). The mere presence of an edge does not guarantee a bonding relationship. A bond is composed of four attributes (Table ). The order attribute ("order") represents the concept of formal bond order, or the number of bonding electrons divided by two. A bond's order may only assume the values one, two, or three. When set to true, the boolean elided attribute ("elided") allows a bond to be omitted during serialization, and to participate in extended bonding. The direction attribute ("direction") is used with other information to conformationally restrict a bond. The two allowed variants for Direction, Up and Down (Table ), will be described in detail later (see: Conformation). The target attribute identifies the child node at which the bond is directed (Table ).