The reason a sash looks like a lady's garment is because it is a lady's garment. Boys, leave it alone, unless you plan on using it as a table runner (or want an extra long scarf for the winter!).

A lot of neophytes to the kilted world will wear a lady's sash with their kilt because they are used to seeing pipers all decked out and wearing a piper's plaid (like this) or depictions of the belted plaid (like this) and they assume that they need something tartan over the shoulder.

But the lady's sash is neither of these. A lady's sash is approximately 10" to 12" in width and usually 90" or so in length. It's also usually of a lighter weight wool. A piper's plaid can be made of 3 or 4 yards of full length tartan, the same heavy weight as the kilt, pleated and sewn into place. It's a beast! And, as I tell people, it's called a "piper's plaid" because unless you are a piper you really don't need to be wearing one!

A fly plaid is something different. It can be found in different sizes, but it's usually a rather large square of tartan, again the same weight as the kilt. It's meant to be evocative of the upper part of the old belted plaid, and has been said here already, is really suited to more formal occasions where a jacket would be worn (like this). I have seen it worn to good effect with a more casual outfit, with a tweed Argyle jacket. But again, some kind of jacket is really needed.

I know I've seen some web sites these days market a fly plaid for casual wear or neo-Jacobean wear, but I just don't think it looks right.

This is not a fly plaid.