What black-tie optional actually means

Floor-length, or a very good midi. Black-tie optional means the host would be delighted if you went full-length, and will not blink if you do not.

What it means

It sits one step below black tie. Black tie means a floor-length gown, full stop. Black-tie optional opens the door to a midi, provided the fabric carries the weight of the room. The test is the fabric, not the hemline: satin, crepe, velvet and anything beaded reads formal at any length. Cotton and jersey do not.

What works

• A floor-length column or A-line, which is never wrong here
• A midi in satin, crepe, velvet or with beading
• A heel. This is the one dress code where a flat sandal reads under-dressed
• Real jewellery, or nothing. Costume pieces read louder under evening light

What to avoid

• A short cocktail dress. It is the one length that reads as a misread of the invitation
• Daytime fabrics — linen, cotton poplin, jersey
• White or ivory if there is a bride in the room
• A day bag. Take something small

One more thing

If you are choosing between a floor-length dress you feel ordinary in and a midi you feel excellent in, take the midi. Black-tie optional exists precisely to allow it.