V. Pants & Waistcoats

The majority of your work building a suit will be complete once you finish the jacket. The other pieces are important, but most of the same decisions will translate from the jacket to them. You'll use the same fabric, the same buttons, the same details, and so on. You'll of course want to make sure they fit properly, but only a minority of the time during your appointment will be spent on their design.

Pants

Your pants pose a set of conditions not experienced by the jacket. A jacket will fit similarly whether or not you're standing or sitting, while the pants will often fit quite differently. The main thing is to ensure they are comfortable no matter your position. It is entirely possible that a pair of pants that appears to fit well while you are standing will bunch up, becoming ugly and uncomfortable the moment you sit down. Test this behavior extensively during your trial fitting. Take a seat, do some silly walks, see how they ride up on a decently long walk, then do it all again with your phone and wallet in your pockets. Put them through their paces before you sign-off and take custody. There are only a couple design elements you really have to worry about ahead of time. The one universal feature is extra reinforcement on the front and back of the pant legs that ensures you always have a solid crease no matter how long you've been wearing them. No one wants to be constantly ironing.

Cut. Do you want straight legs or tapered legs? The straight cut will give you the most wiggle room for any kind of size or weight changes, while the tapered look can look exceptionally stylish if you have the ability to pull it off. If you're unsure, just go with the straight cut.

Length. The length of the legs can make a difference when you're sitting down. Dress pants will naturally rise up when you go from standing to sitting, and the length of the legs will determine if your socks are covered or if you’re showing enough ankle to flabbergast a Victorian. I go with a relatively long length that reaches down to my shoes when standing. This style is necessary because I have thick calves that cause my socks to scrunch, and I refuse to wear socks suspenders to keep them up. If the pants are too short, more leg hair will poke out, and that's not a good vibe in a meeting.

Pleats. You have the option to add pleats around the waistband. These features are created by folding part of the fabric on itself before it is sewn. The result is an increase in the potential volume of the pants. It is more of an old-fashioned kind of look, something your grandfather wore. I'm not a fan, they really distract from the overall shape of the pants.

Cuffs. Another choice for the bottom of the pants is the style of cuff. What you typically see is a clean edge at the bottom, but you can also fold the fabric up over itself to create a little cup. Doing so never seemed necessary to me. Some consider it more fashionable, and it seems to come in and out of the style across the decades.

Fastening. The buttons you pick for the pants will be the same as on your jacket. What you get to pick is where exactly those buttons are placed and how the waistband closes. They're pretty minor differences, you either have a single button in the front that closes like a regular pair of pants or you have a button slightly offset that connects with a tab of fabric with an extra metal clasp on the inside. I tend to go with the latter because it keeps everything in place and creates a cleaner line across the top of the pants.

Belts. Speaking of security, you don't want to be caught with your pants down. The purpose of a belt is to keep your pants up, but a properly tailored suit should not require a belt to stay in place. I also want to reduce the barriers between me and a bathroom visit. There's also the consideration that if you're sitting in the bathroom part of the belt may touch the floor, and that scenario is a nightmare. For colors, the general rule is black for darker suits, brown for lighter suits, and every other color evaluated on a case-by-case basis.

Suspenders. The same logic as belts, suspenders are there to keep your pants up. Suspenders show up quite prominently whenever you take your jacket off, so unless you want to be the suspender guy in the office, you can feel free to give them a pass.

Waistcoats

The first thing to know about vests is that they aren't called vests. The proper term in this context is waistcoat. If you say vest, you'll show just how uncultured you are and it'll be quite embarrassing. The waistcoat is the defining component of the three-piece suit. It increases the flexibility and wearability of the entire ensemble, allowing you to maintain a formal figure without a jacket. Like pants, many of the same design choices will carry over from the jacket. The main consideration for wearing a vest is the time of year. The summer heat precludes three-pieces; however, for very hot weather, you wear one instead of a jacket. Waistcoats can also expand your options for mix-and-match. I like to substitute a linen waistcoat with my blue suit to provide some contrasting color. I recommend having a couple on hand for that reason. Just remember to leave the bottom button unbuttoned. 

Breastedness. The classic vest is single-breasted. Rarely, you may see double-breasted varieties, but I would not recommend them. It adds extra heat retention and reduces its flexibility. The second row of buttons is also be a bit much on a smaller garment.

Pockets. Your standard waistcoat has two pockets near the waist, and they're almost always welted. I wouldn't deviate from that; you don't have a lot of room for anything else. Those pockets though are functional and large enough to carry things like your keys or ID badge.

Lapels. Your typical waistcoat will not have any lapels, simply having a straight cut all the way around to the neck. The minority that do have lapels always strike me as odd. The lapels, by necessity, must be quite narrow, and the miniaturization doesn't look right. They also add a busyness to the overall suit. Wearing a three-piece suit is a complex look in and of itself, and adding more details yields diminishing returns. I would not recommend them.

The Backside. The most important decision you have to make about your waistcoat is what the back will look like. Typically, you’ll see two different materials: the front will be the main suit fabric while the back is made of the suit’s lining. That silkier material helps the jacket slide over it. However, that heterogeneous construction does not work without the jacket.  It makes the waistcoat feel incomplete, like part of your suit is missing. I always have the back of the waistcoat made of the same fabric as the front and haven't noticed any issues with wear. I think this look gives you a more put-together appearance and helps differentiate your suit.

Cinch Strap. The last detail worth talking about is the little belt on the back of some waistcoats. This strap is used to create a tighter fit around the torso. If you have a properly measured and fitting waistcoat, it should neither be necessary nor included.