As the processes of feudal fragmentation accelerated in the 14th century, it became customary for the monarch's sons to receive imperial titles during their father's lifetime; sons were styled co-rulers or junior emperors.
Kui feodaalse killustumise protsessid 14. sajandil kiirenesid, muutus monarhi poegadele tavaliseks saada kuninglikke tiitleid oma isa eluajal; poegi kutsuti kaasvalitsejateks või noor-tsaarideks.