Complex sentence: As was customary for unmarried young ladies at the French court, she was given an honorary style, "Mademoiselle de Chartres", taken from the name of one of her father's appanages.
Split into simpler sentences: As was customary for unmarried young ladies at the French court, she was given an honorary style. She was known as "Mademoiselle de Chartres" after the name of one of her father's appanages.