Shape

A loom is a rectangle, so a rug is a rectangle unless somebody went to trouble. That is the whole explanation for why the other shapes are rare and why they cost what they cost: a round or an oval has to be shaped as it is woven, with the weaver adding and dropping warps at every row rather than working straight up.

Rectangles, which is most of them

Rectangular rugs are the default and the easiest to place, because a rectangle repeats the shape of the room and of almost all furniture. The proportion varies more than people expect: a rug listed as 6x9 and one listed as 7x10 are both "medium", but one is noticeably longer against its width. If a room is close to square, look for the squarer proportions inside the rectangle range before you move to another shape.

Runners

A runner is a rectangle taken to an extreme, typically between 75 and 110 centimetres wide and anywhere from two to six metres long. They are woven for corridors, and they are also the right answer beside a bed, along a kitchen galley and on a long landing. Because a runner takes concentrated foot traffic down a narrow line, the pile matters more here than in almost any other position: this is a job for wool.

Squares

A square rug is uncommon because square rooms are, but where the floor is genuinely square a square rug settles it in a way no rectangle does. They are also the natural choice under a square dining table and in a small entrance hall.

Rounds and ovals

A round rug does two things well: it sits under a round table, and it softens a room built entirely from straight lines. It does one thing badly, which is filling a rectangular floor. An oval is the compromise, longer on one axis, and it suits a long narrow room where a circle would leave two empty corners.

The octagon

The octagon is the rarest shape here and a genuinely old one, cut by the weave rather than by a knife. It reads as a circle from across a room and as something deliberate close up, and it works in a hall or a stairwell where a round rug would look like an accident.

A note on shaped rugs and wear

Any shape other than a rectangle has more edge per square metre, and the edge is where a rug fails first. That is an argument for buying a shaped rug in good condition rather than a worn one, and for looking at the newer end if the rug is going somewhere busy. The age bands set out what to expect from each.