The United States is unique among the Western countries of my acquaintance in that people are sometimes married in the evening rather than the daytime, and that permits an "only in America" style of semi-formal dress for the wedding party which is the substance of today's essay.
Evening black tie does not offer a plethora of options but some are less than ideal for weddings, in my opinion. And if we ignore the completely outrageous, including most of what was worn at this year's Academy Awards, two of the three most common modes of evening clothing are fine for the guests but a little too casual for a wedding party. The top illustration, from the archives of Esquire magazine, shows both of them - the double breasted coat and the descended-from-the-smoking-jacket shawl collar on a single breasted. The groom in particular should be dressed more formally.
The second Esquire illustration shows perhaps the ideal form: the vested, black peak lapelled midnight blue dinner jacket worn with a turndown collar and French cuffed pleated shirt, black bow tie, and undecorated black patent leather oxford shoes.
The classic black tie vest may be the least familiar of these elements to most men. It predates the cummerbund, which was originally only for warm weather wear. It is shawl collared and low buttoning to show more shirt than day wear's familiar six button vest, with but three buttons of its own.
The black tie trouser is worn with braces. It has double pleats, a stripe down the leg that usually matches the jacket's lapels, and is never cuffed.
Now most of these elements may be found ready to wear with diligent searching (the vest is likely to be the most difficult) but generally only in black wool. The more elegant and "blacker than black" midnight blue will probably be found only bespoke but both H. Lesser and Smith's Woolens make appropriate cloth for tailoring. And if the schedule permits, a cost-effective house like the men from Hong Kong will generally be able to deliver exactly what is required for less than the cost of the best ready to wear stuff.
This attention to detail is important, in my opinion. After all, one is hopefully only married one time and the photos last a lifetime or even longer.