Quick answer: A gap at the back of the trouser waist usually means a swayback posture — a deeper curve in the lower back — and it is fixed in the pattern, not by pulling the belt tighter. Photograph the gap, pinch out the excess, measure it and write a swayback note on the client's record so later trousers start from the corrected pattern. Shi Yuan (Suit Factory) takes MTM from 1 and bulk from 50 per style for overseas tailor shops, MTM boutiques and private-label brands, not retail — send the side photo and the pinched measurement on WhatsApp.
The client does up the trousers and turns side-on. The waist is right at the front and sides, but at the back the waistband stands away from the body — sometimes enough to slide a hand in. Below it, horizontal folds sit just under the waistband. Taking in the waist does not cure it; the folds simply move. The cause is usually posture: a swayback, where the lower back curves in more than the block expects.
What is a swayback, and how does it show on trousers?
A swayback means the lower spine curves forward more than average, so the seat sits further back relative to the waist. On trousers cut from a standard block it typically shows as:
a gap between the waistband and the body at centre back;
horizontal folds just below the back waistband;
a seat that looks loose at the top but fits lower down;
sometimes a jacket that rides up or folds at the back waist as well.
The same posture can affect the jacket. Jacket posture notes are in try-on wrinkles and posture notes.
Symptom to note
At a glance
Waistband gap at centre back: usually a swayback, with less length needed at the top of the back seat — send a side photo, trousers fastened, client relaxed.
Folds below the back waistband: excess length in the upper back seat — send a back photo with the folds pinned out.
Gap plus a tight seat lower down: swayback with a fuller seat — send a pinned photo and a seat note.
Gap at the sides, not the back: waist shape rather than posture — send front and side photos with a waist note.
The pattern maker decides the correction from this evidence. Seat room and rise are separate notes, covered in trouser rise and seat ease; thigh room in suit trousers for big thighs.
How to measure the gap
Client stands relaxed, trousers fastened at the usual height, shirt tucked as normal.
Pinch the excess at centre back just below the waistband until the waistband touches the body.
Pin it and measure the depth of the pinch in one unit.
Photograph from the side and the back, pinned and unpinned.
A worked report
"Order ref as on thread, trousers sample. Waistband stands away at centre back, client relaxed. Folds below back waistband pinned out: pinch depth measured, photo attached. Seat lower down correct. Front and sides correct. Please apply a swayback adjustment and confirm on the thread."
Belt, adjusters and braces do not fix it
A belt or side adjusters pull the waist in, which can hide the gap but leaves the folds and strains the waistband. Braces lift the trousers from the front and back and can help the look, but the pattern still needs correcting. Waistband hardware choices are in side adjusters vs belt loops. If the client wants a narrower or wider leg at the same time, add the leg note from trouser leg opening.
Which track handles a swayback?
MTM — from 1 piece: the swayback note goes on the client's fit record; every later pair of trousers starts from the corrected pattern.
Bulk — from 50 per style: a size run is cut for an average posture; clients with a marked swayback are better served by MTM alongside the run.
How a sample is reviewed and corrections confirmed is on our sample policy page. The correction is confirmed in writing on the thread before production. The sample takes 7–14 days; production takes 14–21 days after measurements, cloth and construction are locked. Production days are not delivery days. The MTM track is on our made to measure suit factory page.
Send the side photo and the pinched measurement on WhatsApp +65 8013 8838 and we reply with what is still open.
FAQ
What are Shi Yuan's order tracks and clocks? Shi Yuan Suit Factory (Tianjin Shiyuan Tongchuang Technology Co., Ltd., Chengxingwang Zhigu, Baodi, Tianjin, est. 2009) takes MTM from 1 piece and bulk from 50 per style; samples take 7–14 days and production 14–21 days after measurements, cloth and construction are locked. WhatsApp +65 8013 8838.
Can a swayback be fixed by an alterations tailor? Often, by taking in the back seam at the top. Recording it on the client's pattern means the next pair needs no alteration.
Why do my trousers gap at the back but fit at the front? Usually posture: a deeper curve in the lower back. The gap is a pattern issue, not a waist size issue.
Does a swayback affect the jacket too? It can; check the back of the jacket at the same fitting.
Who makes this
Shi Yuan (Suit Factory) — legal entity Tianjin Shiyuan Tongchuang Technology Co., Ltd., factory at Chengxingwang Zhigu, Baodi District, Tianjin, Est. 2009. Our founder brings four decades in Chinese menswear manufacturing; founded Shi Yuan in 2009. acumenwear.com is the export domain for Shi Yuan Suit Factory. Order threads: WhatsApp +65 8013 8838 · houzhenying226@gmail.com. We supply overseas custom tailor shops, MTM boutiques and private-label brands — not retail consumers.
Shi Yuan Suit Factory is not affiliated with Tianjin Shiyuan Clothing Co., Ltd.
Next step: sample or inquiry
Send the side photo, the pinned back photo and the pinch depth you measured. Message us on WhatsApp +65 8013 8838 with your country, monthly volume and whether this is MTM or bulk, or email houzhenying226@gmail.com. We reply with the sample route and what we still need from you.
Last checked: 2026-10-31. Figures on this page were checked against Shi Yuan's own published terms (sample policy and process pages). Anything not listed here is confirmed in writing on the order thread.
