Cardinal Sin???

[BCAC]

I many ways yes, it's a personal choice. I've certainly seen many people wearing a belt under a waistcoat. But the traditional view is that one either wears one or the other. If your kilt fits you properly, you shouldn't need to wear a belt with a waistcoat, and it would be hidden by the waistcoat anyway. So it would be superfluous. In fact, it would create an unsightly bulge under your waistcoat. So the question which leaps to mind is "why would one want to wear a belt with a waistcoat?"

Now the wearing of a belt over the waistcoat is another matter entirely. It, too, is occasionally done. But those more knowledgeable than I would need to explain the hows or whys of doing that.

Tobus is right, of couse (as usual!). The thing is, now you know WHY you're not supposed to wear your belt with a waistcoat (because of the "supposed" unsightly bulge that it makes), YOU can decide whether you will wear it or not. If you put them both on, it feels comfortable, and there is no "unsightly bulge", then why not wear both? If the belt fits OK and is worn in the right place (at the top of the kilt) then no one will see it anyway. It's not really the right moment to show off that superb buckle, though!

In theory, you're not supposed to need to wear a belt with your kilt if your kilt was made for you. If I'm not wearing a waistcoat, I will always wear a belt because it looks (to me) as if something is missing if there is no belt. With a waistcoat it will depend on the kilt I am wearing. I have one kilt which is now slightly large on my waist (I've lost weight since I ordered it) so if I'm wearing that one, even with a waistcoat, then I'll wear my belt as well. Then there's always the case of the "off the rack" standard sized kilts that some people buy. Because not many people are standard sized, then a belt is often called for (even with a waistcoat).

So all in all, you shouldn't need to wear a belt with a waistcoat but, if you feel the need, or if you want to, then go ahead.

Edited to add: The wearing of a belt over the waistcoat is usually done when (normally in evening dress) one is wearing a dirk, to support the dirk, and then it is called a "dirk belt".