When it comes to choosing clothing to wear under a black maxi dress, there are several options depending on your personal style, the occasion, and the level of coverage or layering you prefer. Here are a few suggestions:

1. Basic slip or seamless underwear: To ensure a smooth and seamless look, you can opt for a basic slip or seamless underwear that matches your skin tone. This will prevent any visible lines or bumps.
2. Strapless or convertible bra: If your maxi dress has a low back or spaghetti straps, you may consider wearing a strapless or convertible bra that provides support without visible straps. Make sure the color of the bra matches your skin tone or the dress.