Sheen is the amount of light a paint reflects once it's dry. Flat reflects almost none, semi-gloss reflects a lot, and eggshell and satin sit in between. Higher sheen means more durability and easier cleaning, but it also shows every roller mark, patch, and wall imperfection. Choosing well is a balance.
Ceilings should almost always be flat. It hides the joint lines and texture variations that overhead light would otherwise highlight, and nobody scrubs a ceiling. Flat also works on low-traffic walls in bedrooms and formal living rooms where a soft, velvety look matters more than washability.
Eggshell is our default for most walls: hallways, living rooms, dining rooms, and bedrooms. It has enough sheen to wipe fingerprints off, but not so much that patches and drywall seams show through. Modern washable eggshells have closed most of the durability gap with satin.
Kitchens, bathrooms, laundry rooms, and kids' rooms benefit from satin. It stands up to moisture and scrubbing. Trim, doors, cabinets, and built-ins get semi-gloss or a dedicated enamel, which is what gives that crisp, furniture-like edge against the walls.
One more rule: the higher the sheen, the more prep the surface needs, because every imperfection will show. That's why we sand and fill before trim and cabinet work more carefully than anywhere else. If you're unsure, we bring samples and look at them on your walls in your light before we commit.