Carpets generally will be 4 metres wide, so measure your area from the widest points, and into doorways to get a more accurate size of the room. Some carpets do come in at also 5 metres, and some in 1,2, 3 and 4 metres - these may not always colour match so try to keep to one width if you have a larger area to do. Carpets with a pattern, even a stripe, will require some pattern matching.
Nylon/ Zylon appeared with stain treatments like the old Antron. A tough material is nylon, but rubbish at stain resistant it had to be treated with Stainguard/ Antron/ Guardmaster and the rest.
Meanwhile polyproylene was advancing. Today it is a fabulous material, inherently stain resistant, well manufactured and keeps its shape.
Because it is hard to dye, it is also bleach cleanable. Gone are the days when it had an artificial streaky appearance, it looks a lot like wool and once fitted hard to tell the difference.
pictures via United Carpets