The rule
If you're genuinely not sure whether the occasion calls for a tie, wear one. You can always take it off and put it in your pocket within a minute of arriving. You can't produce one from thin air.
Why it matters
Being slightly overdressed is a small, forgivable thing that suggests you took the evening seriously. Being underdressed is what you'll still be thinking about at midnight. The tie settles the matter in your favour either way.
How to do it well
- Keep a plain navy or dark knitted tie in your jacket or your car.
- Check what the host will be wearing if you can. It's a perfectly normal question.
- If you take it off, do it without fuss, and fold it away rather than leaving it hanging loose.