Tradition, tradition. Though your fiancé and the male members of the wedding party need not feel too hemmed in, use this formalwear primer as a jumping-off point for creating their looks.
Daytime Informal
Winter
Dark business suit, navy-blue blazer with khaki trousers, navy-blue blazer with cream or white flannel trousers
Plain shirt with four-in-hand tie (aka standard long necktie)
Plain coordinating shoes
Summer
Light-colored suit or navy-blue blazer with khaki, white, or cream linen trousers
Plain shirt with four-in-hand tie
Plain coordinating shoes
Daytime Semiformal
Black or dark-gray sack coat, gray waistcoat
White pleated shirt or soft white shirt with standard four-in-hand tie
Plain black shoes
Daytime Formal
Black or gray morning coat (cutaway) with black- or gray-striped trousers and gray waistcoat
Starched white shirt with turndown collar and a black-and-gray striped tie or dress ascot
Gray gloves
Black patent-leather dress shoes
Evening Informal
Winter
Dark business suit or tuxedo, depending on bride's level of formality
Summer
Navy or light-colored business suit or tuxedo, depending on bride's level of formality
Evening Semiformal
Winter
Tuxedo
Summer
Tuxedo or white dinner jacket with formal black trousers
White pleated shirt with turndown collar
Black vest or cummerbund and matching bow tie or cravat (a short, wide tie)
Black patent-leather dress shoes or plain black calfskin shoes
Evening Formal
Black tailcoat and trousers
White piqué waistcoat
White starched-bosom shirt with wing collar
White bow tie
White gloves
Black patent-leather dress shoes
