How Suit Trousers Are Made: Waistband, Curtain, Fishtail and Seat Seam

Suit trousers look simple next to the jacket, but they are built with as much thought. How the waistband is split, how much spare fabric sits in the seat seam, how the inside is finished and where the lining stops decide how well the trousers hang, how long they last and how much a tailor can adjust them years from now. Here is how suit trousers are made, part by part, and what to check when you buy or commission a pair.

How are suit trousers made?

Each leg of a pair of suit trousers is cut from two panels: a front and a larger back that covers the seat. The maker builds the pockets and the fly, joins the long seams of each leg, attaches the waistband and its curtain to each half, and joins the two halves last along the seat seam. Building trousers this way is what makes the waist and seat easy to adjust later: the seam that closes the trousers is also the seam a tailor opens.

What is a split waistband?

A split waistband is divided at the center back by a seam that runs straight down into the seat seam. The two halves of the waistband are separate pieces, so a tailor can open the back, take the waist and seat in or let them out together, and close it again without touching the front. Most well-made suit trousers have one. A one-piece waistband, common on cheaper trousers, is much harder to alter.

Inside, the waistband is stiffened with an interlining so it holds its shape and does not roll over. Many ready-made trousers add rubber grip strips to hold a tucked-in shirt in place. At the front, an extended tab closing with a hook and bar, plus an inside button, takes the strain off the fly and gives a clean line under a jacket. Some tailored trousers also have a small V-shaped opening at the top of the center back seam, a neat finish where the two halves of the waistband meet.

What is the seat seam, and how much can it be let out?

The seat seam is the center back seam of the trousers. It runs from the top of the waistband, down over the curve of the seat, to the crotch. On good trousers the maker leaves a wedge of spare fabric, the inlay, folded inside it: widest at the waistband and tapering to almost nothing near the crotch. That inlay is what a tailor uses to let the waist and seat out.

Well-made suit trousers usually leave enough inlay to let the waist out by about 1 to 1½ inches (2.5 to 4 cm), and bespoke makers often leave more. Cheap trousers trim the seam to a narrow overlocked edge, which leaves nothing to let out. Taking in is easier: about 1 to 2 inches (2.5 to 5 cm) is routine before the back pockets start to crowd together.

What is a trouser curtain?

The curtain is usually made of strong cotton pocketing, often called silesia, or of lining fabric, and on good trousers it is split at the back with the waistband so the waist can still be altered. It hides the raw edges and the tops of the pocket bags, and it is the part of the waistband that sits against your shirt. A neat, well-sewn curtain is a small sign that the inside of the trousers was made with care.

What is a fishtail back on trousers?

A fishtail back is a raised back waistband, split at the center in a V, with each side rising to a point so the top of the trousers looks like the tail of a fish. It is made for braces (suspenders). Braces fasten to six buttons: two pairs at the front and one button on each side of the split at the back. The extra height at the back carries those rear buttons up toward the braces and keeps the shirt covered when you bend or sit.

Fishtail backs come with a higher rise and usually no belt loops, since you do not wear a belt with braces. Because braces hold the trousers from the shoulders, the legs hang straight and the waist can be cut with a little more ease. You will find fishtails on bespoke trousers, on many formal trousers and on some traditional ready-made pairs.

Our guide to belt loops, side adjusters and braces helps you choose, and pleated vs flat-front trousers explains how pleats and rise work together.

Pockets, fly and fastenings

• Front pockets. Most suit trousers have pockets set into the side seams, either straight down the seam or slanted toward the front. Straight on-seam pockets give the cleanest line.
• Back pockets. Usually one or two jetted pockets, a narrow slit finished with fine strips of fabric, closed with a button or a button tab. Some formal trousers have none.
• Fly. A zip is now usual, but many bespoke makers still offer a button fly, which lies flat and is easy to repair.
• Pocket bags. Strong cotton twill, because pockets take daily wear. Small bar tacks, tight blocks of stitches, reinforce the corners of the pockets and the base of the fly.
• Small extras. Some trousers add a small watch or coin pocket at the front of the waistband.

Are suit trousers lined?

Most good suit trousers are lined at the front to just below the knee. The knee lining reduces friction on the inside of the fabric, helps the legs hang straight and cuts down on bagging at the knee when you sit. Trousers in scratchy or heavy winter fabrics are sometimes fully lined, and summer trousers are often left unlined for coolness. Many trousers also have a reinforcing piece of lining or cotton at the crotch, where the fabric wears fastest. Our guide to suit linings covers the materials.

How are trouser hems finished?

The hem is turned up inside and stitched with a small stitch that barely shows on the outside. A plain hem can be straight, or slanted so the back sits about ½ to ¾ inch (1.3 to 1.9 cm) lower than the front and covers the top of the heel. Turn-ups (cuffs) are usually 1½ to 1¾ inches (3.8 to 4.5 cm) deep and suit business and casual trousers, not black tie.

Two details mark a careful hem. The first is a heel guard, a strip of tape sewn inside the back of the hem where your shoe rubs, which saves the fabric from wearing through. The second is spare length: about 1½ to 2 inches (4 to 5 cm) left folded inside rather than cut off, so the trousers can be let down later. Our guides to hemming suit trousers and trouser cuffs cover the choices.

How to check trouser construction in the shop

Ask to turn the trousers inside out. A minute with the inside tells you more than the label.

Construction sets what is possible; fit decides how the trousers look on you. Read how suit trousers should fit for the seat, thigh and length, and what a tailor can alter before you buy a pair that needs work.

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