A single sporran for very casual to semi-formal?

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While military regulations may specify a black bow tie with mess dress when attending a white tie event, civilians should follow the conventions of wearing a white tie (or, depending upon the style of the jacket, a lace jabot) when in Highland attire. The idea that black tie could be worn in lieu of white tie dates from the end of WWII when His Majesty, King George VI, for a brief time relaxed the white tie requirement at dinners when members of the royal family were present. This was done so that those not possessing formal evening attire due to war-time and post-war shortages could still attend.

Except that the dress code for the Royal Caledonian Ball explicitly says black bow tie for highland attire (or lace jabot) and it is clearly, based on the non-highland dress code, a white tie event.

Dress

Part of the particular appeal of the Ball is that everyone attending is wearing traditional formal evening dress. The dress requirements are stated below in summary and in detail.

Please note that those not correctly dressed will be refused entry.

If you are unsure about any aspect of the dress code, please contact the Ball Secretary.

In summary

Ladies
Full length dress

Gentleman
Highland evening dress, or
Evening tails with white tie, or
Mess kit
No dinner jackets

In detail

Ladies - essential
Floor length evening dress, or
Mess dress (for those entitled to wear it) with full length skirt

Ladies – optional (but encouraged)
Tiaras
Clan tartan sashes
Orders and Decorations

Gentlemen - essential
Highland evening dress comprising kilt and sporran, evening jacket of black broadcloth or coloured velvet, white evening shirt and black bow tie or lace jabot, or
Evening dress, comprising black evening tail coat, white shirt with wing collar, white bow tie and white waistcoat, or
Mess dress, worn according to regulations, with, preferably, where permitted, any more formal variation such as stiff shirt, wing collar and black bow tie

Gentlemen – optional
Orders and Decorations

For the set reels gentleman taking part must wear highland evening dress or mess kit, tail coats only if part of mess dress, and ladies must wear clan sashes.