Two pairs of blue jeans can make the same top and shoes look like completely different outfits. The reason is rarely the wash or the brand. It is the leg shape: how wide the jeans are at the thigh, at the knee and at the hem, and where they stop. Straight, wide and barrel are the three shapes you see most right now, and each one works by a different rule.
Below you can swap the shape and the hem length and watch what changes. The numbers under the drawing compare the hem opening with the thigh, which is the quickest way to tell the shapes apart in a shop.
90%
Thigh
A straight leg is about the same width at the thigh and the hem, so the eye reads one tall column. That makes it the easiest to pair with almost anything: a fitted knit, an oversized shirt, a boxy jacket. If you own one pair of jeans that goes with everything, this is usually the shape.
The details that separate a good straight pair from a forgettable one are small: a firm, mid-weight cotton that holds its line, a hem that ends cleanly at the shoe, and a rise (the distance from the crotch seam to the waistband) that sits where you actually wear your waistband.
Wide-leg jeans open up from the knee, so the base of the outfit becomes the broad part. The classic way to balance that is with something more compact above: a cropped jacket, a tucked tee, a fitted knit. Length is the rule to remember. A wide leg that stops short looks like an accident; one that reaches the top of the shoe looks intended.
Wide on the bottom, compact on top. Or the other way round, but never wide everywhere.
Shoes with a bit of sole, such as a chunky loafer or a flat trainer, stop a long hem from dragging, and you can still see the toe peeking out.
Barrel jeans, sometimes called horseshoe or bow-leg jeans, bow outward at the thigh and knee and narrow again towards the ankle. Because the widest point is in the middle, the shape stands out even in a plain wash. It is easiest to wear with a simple top, and ankle or slightly cropped lengths show the curve at its best.
One thing to check in a shop: the curve should sit where the pattern intended it. If you pick a size to fit the waist and the leg looks straight rather than rounded, try the next size up, or a different cut.
A dark, even wash makes any shape look more formal; a light or faded wash makes it more casual. Stiff, heavy denim keeps a curve or a wide leg crisp, while soft, stretchy denim collapses into a softer line. If you want the shape to be obvious, choose a firmer fabric. If you want it relaxed, choose softer.