There was a period that there was no reigning monarch between the execution of Charles I in 1649 and the restoration of Charlies II in 1660 as result of the English Civil War. It was the war between the supporters of an absolutist king; Charles I and the supporters of the parliament. It led to execution of Charles I and replacement of English monarchy with the Commonwealth of England etc.
The supporters of the Parliament was called 'Roundheads', this nickname came from their closely cropped hair and helmets were also round shape along the line of head. The supporters of the king was called 'Cavaliers'. As contrasted with Roundheads, they had long flowing hair in ringlets, brightly coloured with elaborate trimmings and lace collars and cuffs and plumed hats.
Big Mouse; 'You are Cavaliers.'
Small Mouse; 'Ah, that's because I am gorgeous.'
Big Mouse; 'No, because your head's shape is not round (backside of the head is flat)...'
Small Mouse; '.......'
Big Mouse; 'I am Roundheads.'
Small Mouse; 'You know what. Cavalier is expensive to maintain.’