The Rose Garden Puzzle

“A friend of mine,” said Professor Rackbrane, “has a rectangular garden, and he wants to make exactly one-half of it into a large bed of roses with a gravel path of uniform width around it. Can you find a general rule that will apply equally to any rectangular garden, regardless of its proportions? All the measurements must be made in the garden. A plain ribbon, no shorter than the length of the garden, is all the material required.”