An undershirt stays out of sight when its shape cooperates with the clothing above it. A pale color cannot hide a crewneck beneath an open shirt, and a thin fabric cannot solve sleeves that extend beyond a short-sleeved top. Begin with the outer garment you want to wear.
Put the undershirt on and then add that shirt. Open the top buttons to your usual position. Look from the side and slightly above as well as straight ahead; the neckline can appear during normal movement even if it is hidden in a static mirror view.
A deeper V may work beneath an open collar, while a crewneck may be preferable when visible layering is intentional. The right choice depends on the opening of the outer shirt and how much coverage you want. There is no reason to choose a very deep neckline if it creates another comfort problem.
Edges matter as much as fabric
Check sleeve hems beneath the outer sleeves. Thick seams can create a ridge even when they are fully covered. A sleeveless underlayer avoids that edge but changes coverage around the armhole. Try both only if they serve the role you need.
At the waist, move as you would during the day. An undershirt that is too short may come untucked and bunch under the shirt. An excessively long one can gather inside trousers. The desired length depends on your rise and whether you intend to tuck it, so compare a familiar garment rather than relying on a generic longline label.
Color contrast is another variable. An underlayer close to your skin tone can be less visually prominent through a light shirt than bright white, but texture and seams still show in some fabrics. Test the combination in good light. Do not buy a large pack before confirming that one works with your actual clothes.
Feel the complete outfit while seated and reaching. An undershirt that is invisible but restrictive is not doing its job well. Read the care label too, particularly if you expect to wash it with ordinary T-shirts. Choose a shape and finish that solve the specific layering problem, then consider duplicates once you know it suits your routine.
