[the420skipper]

I agree that odds are, the host is thinking suit and tie. But unless you ask him (which would be ideal, obviously) you can't really be sure of that, and showing up in a suit when everyone else is in dinner jackets would be worse than the other way around, I think. Moreover, I don't know the crowd you're dealing with, but it's quite likely that at least some men will show up in black tie. So even though it's quite unclear, it's always better to be overdressed than under.

 

[the420skipper]

I heartily disagree with this assessment and suggestion. It is not always better to be overdressed than under. Nor is it always better underdressed than over. For business and casual social settings, being dressed one step up is fine and plays on the safe side. But this assumes you know it involves two close categories and you've chosen the step up.

Once you get into black or white tie territory, those categories are so distinct that one step up isn't so simple. If the question is whether the event is black or white tie, I'd suggest the lesser of the two to play it safe. Between black tie and dark suit I'd choose dark suit. These are almost always acceptable alternative as they respectfully represent the wearer's best attire available and won't overshoot the host, guest of honor, or wedding party. (Note: This applies to the situation of the opening post and not to events where I know both dress categories will be present.)

pbc

My suggestion was based on the idea that the groom and at least some of the wedding party would be wearing black tie... but I suppose that's not really a fair assmuption.

Having read other people's viewpoints, and thinking about it more, it does seem unlikely indeed that the invitation means black tie. After all, if that's what they meant, assuming this is not a very old-fashioned invitation, why wouldn't they have just said black tie? Regardless of his sartorial knowledge, anyone would know that black tie is a much more specific and facile way to express the dress code requiring a dinner jacket than "semi-formal".