In order to solve the problem of succession Diocletian created this system by which two co-emperors each adopted a junior caesar to succeed them.
Citation:
Bouchard, Elsa. "Establishment of the tetrarchy".
The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11529, accessed 19 June 2013.]