The French Connection
Genealogies had implications in not only domestic power struggles, but also across the Channel in connection with English claims in France. Assertions in the 15th century that English kings had a right to the French throne originated in Edward III’s status as the grandson of Philip IV of France (Edward was the son of Philip’s daughter, Isabella of France).
On the Canterbury Roll the French and English royal bloodlines are linked through the marriage of Isabella of France to Edward II of England.
A blue line extends from the French king Philip IV "the Fair" (Pulcher) (CRN601) linking the French royal line with Edward II of England (CRN492) via Philip's daughter (CRN609).
Isabella of France was the daughter of Philip IV and Joan of Navarre. She married Edward II in 1308 becoming Queen Consort of England. After the death of Isabella's three brothers without surviving children, her cousin, Philip of Valois, became king of France as Philip VI.
The English claimed the right of succession through the female bloodline. From an English perspective, the crown of France should have passed to Philip III's great-grandson, Edward III of England, because this was the most direct line of descent; from a French perspective, Philip VI was the rightful king because women were excluded from succeeding to the French crown.
A simplified genealogy of the two royal families illustrates the English claim.
The English Claim to the French Crown
To make the point visually obvious, the Canterbury Roll changes the appearance of the central axis between the reigns of Edward II and Edward III. Whereas prior to Edward II this had been a single red line, from Edward III the axis becomes an alternating mix of red and blue that conveys the essence of the English claim to the French kingdom.
The central axis representing the succession of kings of England incorporates blue, signalling the English claim to the French throne from Edward III's reign.
Although the Canterbury Roll's primary focus appears to be the Lancastrian–Yorkist conflict, the care taken to colour-code the French claim suggests that there is an unspoken consensus that English kings possessed a "right" to the French throne.