In 1923, according to a survey of 300 men in Palm Beach, Florida by the now-defunct Men's Wear, 76% wore white odd trousers or plus fours in flannel or linen, 21% wore suits, 1% wore gray flannels and 2% wore knickers in other fabrics. Trousers, particularly the gray flannel variety, have covered a lot of ground in the intervening 85 years, and we've substituted shorts for plus fours and knickers along the way.
But this essay is on trousers and there are two basic types with countless variations. For my taste, the dressier of the two are high waisted English style trousers cut to be worn beltless, with side tabs or braces (the straps that Americans call suspenders), and pleats in the front and cuffs on the bottoms. Braces let trousers hang straighter and don't require adjustment during the day. They are dressier because the high waist sits above a mature man's paunch and lets the trousers hang in a straight line to the shoe. Needless to say, this allows for a sleeker appearance than does a suggestion of stomach protruding over a belted waistline. The negative is that they should be covered by a vest or jacket at all times even if your name is Larry King.
The other principal type of trouser is flat fronted, cut to be worn belted on the hips, and usually without cuffs. This style was originally laborer's garb but it's been elevated to the status of art by continental tailors who prefer it for suits cut for men with trim figures. It's a fine look as long as the wearer is careful that the open quarters of his jacket don't expose his belt buckle and blind pedestrians with the flash.
I prefer high waisted trousers with suits but either belted or beltless trousers are reasonable choices with odd jackets and for more casual wear. There are several options for keeping up beltless trousers, including side straps instead of tabs, and the DAK waistband, however these are usually difficult to find ready to wear.
Button cuffs may be the most obscure trouser detail. Instead of stitching the turned-up cuff to the trouser leg, the tailor sews a button to the inside of the cuff and a buttonhole on the seam of the pant leg. Each cuff has two buttons, one on each side. This feature can safely be left to men who commonly need to brush debris out of their trouser bottoms.
Finally, we should consider the fly, and that's usually closed by a zipper, a relatively modern invention that no longer jams like it used to. The older alternative is the button fly, normally eight buttons that pretty well guarantee that a man will be fumbling with himself at a urinal long enough to draw nervous glances from those around him. The Duke of Windsor preferred the zipper and that's good enough for me.
Trouser fabrics are widely available in much more variety than was seen in Palm Beach those many years ago. Flannel, linen, corduroy, cotton drill, and fresco are some of the common odd trouser types. For some thoughts on an odd trouser wardrobe, see my January essay here.