What is the lining of your underpants through trousers?

The lining of your underpants is typically not visible through trousers. However, if you're asking about how underpants may affect the appearance and fit of trousers, the type of fabric, color, and fit of the underpants can influence how the trousers drape and fit. For a smoother look, it's often recommended to wear seamless or nude-colored undergarments to minimize any visibility. If you meant something else, please clarify!