In the english civil war why was the roundheads and cavaliers named those names ?

The Roundhead were Called Roundheads because they wore round hat
The cavaliers were called the cavaliers because they were good with horses

Cromwell supporters mostly had their hair closely cropped around the head, hence the name roundheads.
Cavaliers, supporters of the King, on the other hand wore long hair, as this was the fashion at the time. They were named after french word chevalier, which meant "horseman".

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