I know this topic has been extensively discussed in another section, but just wanted to add that the view here in Scotland is that with the traditional kilt the pin should no nay never be used to pin the two aprons together. Here the view is that the pin serves 2 basic functions (a) as decoration and (b) as a weight on the bottom corner to help hold down the top apron.

For decorative purposes it can take almost any form that takes your fancy, although the for semi-dress/formal occasions two most popular are probably variations on the sword theme and round celtic brooches. However, for casual kilts where the emphasis is on informality the favourite is probably still the original "large safety pin" type.

In order to be of any use as a weight, the pin needs to be near the "open" corner, about 1-1.5ins in/up from both edges is about ideal. I have seen them worn in the range 3-6ins up, but this is really too far to serve any practical purpose.