In heraldic rules, two arches (or four half arches) signify a king; four arches (or eight half arches), an emperor.

When Queen Mary was crowned, she decided to have the four arches to reflect her status as Empress of India. The design was closely copied when, later in the same year, a crown was made up for George V to wear at the Delhi Durbar. There was a huge amount of controversy over this latter crown, which is worthy of a thread in its own right!

Queen Elizabeth was shown both two- and four-arch designs at the beginning of 1937, opting for two from personal preference. However, pre-Coronation sketches of the forthcoming event do show her being crowned with a four-arch crown similar to that worn by Queen Mary.

A final thought on arches is that the coronet of the Heir Apparent has one arch, both in heraldry and in life!