So, I've been quite curious about how the sgian achlais would be worn. Everything I've read just says that it was carried in the sleeve, or under the arm near the armpit. Would a sheathe be physically sewn into a garmet to hide the knife, or was it strapped on to the arm somehow (like a modern day sling for a concealed weapon). Any thoughts? I know it's a bit of a mystery. This is of course assuming that the sgian achlais is the "black knife" that purportly evolved into the sgian dubh and not the knife in a hunting trousse. Thanks.

Some people wear the sheath suspended from a long leather thong over the shoulder. I don't know much about Scottish garments of the 17th and 18th centuries, but a pocket in the jacket seems like a good place for the knife/sheath. Think of a modern suit jacket's inside pocket and move it back a little. Maybe surviving garments don't have such a pocket, but the idea seems practical. :)

I don't know that we'll know conclusively. There aren't many references to sgian achles; I can only think of the Edward Burt quote that everyone cites. Also, only a small number of surviving knives are even tentatively called sgian achles.