The rules of formal trouser dress
For black tie, the dress trouser must have a single silk braid (not double, which is white tie) down the outer seam of each leg. No belt loops — the trouser is worn with braces. No turn-ups — the hem is clean. The break is minimal: the trouser falls straight to the shoe with almost no fold. The cloth matches the dinner jacket, typically barathea wool or a midnight blue.
For morning dress, the trouser is in a specific black and grey stripe — the striped trouser that is one of the most recognisable elements of formal daytime dress. It is worn with braces, with no turn-ups, and with a clean break at the shoe.
For a formal matched suit worn to a serious professional occasion, the dress trouser may have side adjusters rather than belt loops, a half-inch of silk braid at the seam (a subtle formal detail), and a precise break appropriate to the occasion.