Dress trousers are formal trousers, and what counts as correct depends entirely on what you are wearing them to. Black tie means a single silk braid down each outside seam and no turn-ups. Morning dress means the particular grey and black stripe. A formal matched suit means the trouser comes from the same cloth as the jacket, cut to a slightly dressier specification than an everyday one. Each of these has its own rules, most of them older than any of us, and we make them to those rules rather than to whatever is convenient.
What each occasion asks for
For black tie: one silk braid down the outside seam of each leg. One, not two. Two braids is white tie and you will be the only person in the room who has got it wrong. No belt loops, because the trouser is worn with braces. No turn-ups, so the hem stays clean. The break is minimal, with the trouser falling almost straight to the shoe. The cloth matches the dinner jacket, so usually barathea or a midnight blue.
For morning dress: the black and grey striped trouser, which is one of the most recognisable pieces of formal daytime clothing anywhere. Braces again, no turn-ups, and a clean break at the shoe.
For a formal matched suit going somewhere serious: side adjusters instead of belt loops, sometimes a narrow half-inch of silk braid at the seam as a quiet formal note, and a break cut precisely to the occasion. Tell us where you are wearing it and we will set the details accordingly.