For many centuries, the title "princess" was not regularly used for a monarch's daughter, who might simply be called "Lady" or a non-English equivalent.
Old English had no female equivalent of "prince", "earl", or any royal or noble title aside from queen. Royal women, or women of the nobility, were simply addressed or referred to as "Lady" or "The Lady Firstname" for a particularly highborn woman.
Photography © Tuyen Tran