A bra shows under a T-shirt for one of three reasons, and knowing which one you have makes the fix obvious.

The first is seams. Most bras are built from cut-and-sewn panels, and every join creates a raised line. Under a fine jersey tee that line prints straight through, usually as a diagonal across the cup or a horizontal under the bust. A moulded cup has no seams at all: the fabric is heat-formed into a single shape, which is why it stays invisible. This is the single biggest factor and the one that most people never think about.

The second is edges. A lace trim, a picot edge or a decorative scallop all have a different thickness to the fabric beside them, and that step shows as a visible outline. A bonded or laser-cut edge, which finishes without a hem, does not.

The third is fit, and it is the one that produces the effects people blame on the tee. A band that is too loose rides up and creates a bulge at the back. A cup that is too small pushes tissue over the top edge, which reads through a T-shirt as a line across the chest. Neither is a fabric problem, and no smoother bra will fix either.

So the short answer: moulded cups, bonded edges, correct band.

These terms get used loosely, so it is worth separating them. A moulded bra has cups formed under heat into a fixed shape, so there is no seam across the cup. A seamless bra means the same thing in most brand language, though some use it to mean the band and wings are bonded too. A T-shirt bra is a marketing category rather than a construction, and it almost always means moulded, lightly lined and neutral in colour.

Lining is a separate decision from moulding. A lightly lined cup has a thin layer of foam that smooths the nipple line and holds shape; an unlined cup has none, which is more breathable and reads more naturally under a heavier fabric but shows more under a fine one. Neither is better. Lightly lined is the safer default under a thin tee.

Underwire is about support, not visibility. A well-fitted wire sits flat in the root of the breast and does not show. A wire that sits on breast tissue, which is the usual sign of a cup that is too small, digs and creates a visible line. For smaller busts a wireless moulded bra or a lined bralette often works better under a tee than a wired one, because there is less structure to print.

Colour is the last lever, and the easy win. A bra close to your skin tone disappears under white and light colours. White under white is the most common mistake there is, because it reads as a bright block through thin fabric.

Do this in the fitting room, in the mirror, wearing a thin plain top over the bra. It takes less time than reading a size chart.

Check the band first, because everything else depends on it. Two fingers should slide under it with slight resistance, and it should sit level all the way around rather than riding up at the back. On a new bra you should be fastening the loosest hook, so there is somewhere to go as the elastic relaxes over its life.

Then check the centre panel between the cups. It should sit flat against the sternum. If it lifts away, the cup is too small, and that is the fault behind most of the lines people see through a T-shirt.

Then raise your arms and roll your shoulders. If the band rides up or the cup gapes, the fit is wrong regardless of the label size.

Finally, look at yourself sideways with a flash photograph. It is unflattering and completely reliable, and it will show a seam or a cup edge that soft fitting-room lighting hides entirely. If nothing prints in a flash photo under a thin tee, nothing will print in daylight.

What kind of bra does not show under a t-shirt? A moulded, lightly lined bra with bonded edges, in a colour close to your skin tone. Seams and lace are what show, not the bra itself.

What is a t-shirt bra? A marketing term for a moulded, lightly lined, smooth-cup bra in a neutral colour. It describes the intended use rather than a specific construction.

Do bralettes show under t-shirts? Unlined ones can, because there is nothing smoothing the nipple line and the edges are often lace. A lined bralette with a bonded edge is close to invisible.

Should I wear a nude or white bra under a white shirt? Nude, or whatever is closest to your own skin. White is more visible through thin white fabric than a skin tone is.

Why is my bra showing a line across my chest? Almost always a cup that is too small, which pushes tissue over the top edge. Going up a cup size and down a band size is the usual fix.

Is a wireless bra less supportive? Not automatically. A well-constructed wireless bra with a firm band supports through the band rather than the wire, which is where most support comes from anyway.