These words refer to a shape’s outside boundary, taken as a whole. Perimeter can refer both to the bounding lines that enclose a plane figure and the total length of this boundary: He posted No Trespass signs along the perimeter of his estate; the ins and outs of fjords that add hundreds of miles to the perimeter of the Norwegian coastline. The word can also refer to the outer edges of something far less clearly defined: the shifting perimeters of each task force in the battle zone. Periphery relates most closely to this last sense of perimeter , referring more vaguely to the outer edges of something, especially as seen from within: a planet on the periphery of the camping area. Sometimes the word is seen as applying mainly to solid objects, but this is not always true. The word does have a more generalized use to suggest something that is far from the centre of things and thus of little importance: an official position that was on the periphery of the administration’s centre of power.

Circumference refers to the perimeter of a circle, both to the line itself and to its total length. By extension, the word can refer to the bounding line of any curved shape, plane or solid: a hundred seats along the outside circumference of the amphiteatre; a barrel that was four feet at its widest circumference . Girth refers specifically to the circumference of a curved solid, particularly the waist or belly of a person or animal: the girth of a red gum at its base: a portly man of astonishing girth ; a strap too short for the girth of the horse. In relation to the last example, girth is applied also to the strap itself, which is fastened round the belly of an animal to secure a saddle or pack.