Black-tie optional means that guests have the option of wearing their most formal attire (think: an evening gown or tuxedo) but can also choose to wear a formal suit or cocktail dress instead. Basically, you have some flexibility to play with your fit. Generally, a black-tie optional dress code means that the couple and their wedding party will be dressed in black tie. It's therefore courteous for guests to reflect this level of formality in their own attire. You can either go full-on glam in black-tie-worthy attire or opt for a slightly more relaxed but still formal ensemble.