The term four-in-hand is of often seem in invitations, especally if military people are attending. For example " Dinner at 7 pm, dress is informal, suit and four-in -hand tie acceptable. This tells a gentleman he should wear a dinner jacket (ie. Tuxedo) with informal white shirt and black bow tie, but men without dinner jackets may attend but should wear a conservative business suit and necktie. No sport jackets allowed. It also tell women that dinner dresses can be worn, but tea dresses are acceptable. There has been a trend in the last 20 years to wear tea dress and save the dinner dress for semi formal affairs.