“In today’s web, web pages have unidirectional links - so document A links to document B, but not vice versa. Unidirectional links were a key factor in simplifying early web prototypes so that they could be feasibly implemented. However, this type of link limits the ability to trace back to related material. Bidirectional links connect two documents using a two-way link that can be followed from either document. Designs with bidirectional links, such as Ted Nelson’s project Xanadu, were too complex to successfully construct.”

June Jamrich Parsons, “New Perspectives on Computer Concepts 2016, Introductory” (p.240)