Always balance the space placing a rug equidistant from the perimeter of a room. This rule is a must with almost no exceptions and is essential when creating a balanced and proportionate space.
Layering & scale
If you are working with an open plan space then think carefully about how you zone areas of the room. Area rugs can be very effective in creating designated spaces and defining their uses.
Mind the gap
A visual space between the wall and the rug showing the floor should always be left around the edge of the room. This can vary from 10cm - 40cm but should feel consistent and be even. If furniture lines the perimeter the rug should cuff in front of it. Never put items half on or half off.
What generally works for one space will not always work for another but proportion is they key. Here are an array of classic layouts based on standard rug sizes which set the standards.