There is only one height
The temptation is to centre the work on the wall. The result is almost always too high. The right reference is not the wall, it is the eye: centre of the work 145-150 cm from the floor. If your room is one you mostly sit in — a living room, a dining room — drop it a few centimetres.
Above furniture, a second rule applies
When there is furniture below, the work enters a relationship with it. Leave 20-30 cm between the top of the furniture and the bottom edge of the work. Less, and they read as one block; much more, and the work floats.
The right wall and the wrong one
Avoid: the wall facing a large window, where reflections make any glazing unreadable; the wall above a radiator or fireplace, where heat and air movement dry the paper; exterior walls in damp houses, where condensation forms behind the frame. A one-centimetre gap between frame and wall, made with two adhesive bumpers at the bottom, solves most of that.
Composing several works
For a group, decide the axis first: either all aligned along the top, or all centred on the same horizontal line. Then keep a constant gap between pieces — 5-8 cm for formats like 45 × 30 cm or 60 × 40 cm, more only if the works are large. The group as a whole follows the same rule as a single work: centre of the composition at 145-150 cm.
Light: how much and from where
An adjustable spot at 30 degrees to the wall lights the work without bouncing the glazing into the viewer's eyes. A narrower angle creates a reflection; a wider one throws the frame's shadow onto the work.
On temperature: 2700-3000 K (warm) for warm-toned work and painting, 3500-4000 K (neutral) for black and white, which goes muddy under light that is too warm. A high colour rendering index (CRI 90 or above) changes the result more than wattage does.
The light that ruins
Direct sun is the fastest way to fade a work on paper, and there is nothing to be done once it happens. Artificial light too, over twenty years, leaves its mark. If the room is very bright, UV-filtering glazing is not a luxury.