Social events where all attendees are expected to wear formal attire. Both males and females attending a black tie affair make efforts to comply with the dress code
and choose clothing that is considered appropriate formal dress for the time of day, the season, and the nature of the event. A black tie affair is different from a semi-formal event, where there is a wider range of clothing options considered to be appropriate semi-formal attire.
I heard someone in our neighborhood was holding a black tie affair.