A tailored jacket is one of the more complex garments in ordinary use, and almost all the complexity is inside where nobody looks.
The canvas
Between the outer fabric and the lining, across the chest and lapels, sits a layer that gives the jacket its shape.
Full canvas means a layer of horsehair or similar material, stitched in, running the length of the front.
Half canvas means it covers the chest and lapels but not the full front, with fusing below.
Fused means glued interfacing throughout, with no canvas at all.
The difference in cost is substantial because canvas must be attached with stitching, some of it by hand, and it must be shaped to the wearer.
Why it matters in use
A canvassed jacket moulds to the wearer over time. The canvas is flexible and stitched loosely enough to move independently, so it takes the shape of the chest.
A fused jacket cannot do this. The glue holds the layers as one, permanently, in the shape it was pressed into.
Fusing also fails eventually. Repeated cleaning and moisture break the bond, and the result is visible bubbling across the chest and lapels that cannot be repaired.
Modern fusing is far better than it was and the failure mode still exists.
How to tell
The pinch test. Take the front of the jacket between thumb and finger, below the buttonhole and away from the seam, and separate the layers.
If you can feel three distinct layers with something floating between the outer fabric and the lining, there is canvas.
If it feels like a single board, it is fused.
Do it on the lapel too, since half-canvassed jackets have canvas there and not lower down.
The lapel roll
A canvassed lapel curves outward in a gentle roll from the collar to the buttonhole.
A fused lapel is pressed flat, with a crease rather than a curve.
Which is visible from a distance once you know to look, and it is one of the reasons some jackets look soft and others look like signage.
The shoulder
Where construction philosophies diverge most visibly and where fit matters most.
Structured shoulders use padding and a sleevehead roll to create a defined line.
Unstructured shoulders use minimal or no padding, letting the fabric follow the body.
A shoulder that is too wide cannot be fixed. The seam must sit at the edge of your shoulder, and altering that means rebuilding the jacket, which costs more than most jackets are worth.
This is the one measurement to get right before considering anything else.
What can be altered
Worth knowing before buying.
Sleeve length, easily, though jackets with functioning cuff buttonholes are harder and more expensive.
Waist suppression, taking in the sides, within a limit set by the seam allowances.
Jacket length, sometimes, depending on the pockets and the vent.
Shoulders, essentially not.
Chest, not meaningfully.
Which means buying for shoulders and chest and altering the rest is the correct order.
The trouser side
Less structurally complex and with more alteration scope.
Waist, seat, taper and length are all adjustable within reason.
The rise — the distance from crotch to waistband — is not, and it determines where the trousers sit, which changes the whole proportion.
Low rise with a long jacket looks different from high rise with a short jacket, and neither is wrong, and mixing without noticing is common.
Where the value sits
A half-canvassed jacket that fits, altered properly, will look and last better than a full-canvassed jacket that does not fit.
Alterations cost a fraction of the garment and are the highest-return spending in the whole category.
Most people I know who bought one good jacket and had it altered properly wear it more than everything else combined, which is the practical case for the whole approach.
Vents
A detail with a functional origin that has become stylistic.
A single vent at the centre back is the most common and it spreads open when sitting or when hands go in pockets, which is not flattering.
Double vents at the sides allow the jacket to move without gaping, and they cost more fabric and more work.
No vent produces the cleanest line and the worst behaviour when sitting.
Pockets and buttonholes
Patch pockets are stitched on, which is casual and the simplest.
Flap pockets are set into the fabric, which requires cutting into the front and finishing the opening precisely.
Jetted pockets have no flap and a finished slit, which is the most formal and the most difficult to execute cleanly.
Buttonholes are worth examining. A machine buttonhole is uniform and slightly flat. A hand-worked buttonhole has a raised gimp cord under the stitching and a rounded eye at one end.
The lapel buttonhole is the one to check, since it is the most visible and the one makers put effort into when they put effort in anywhere.