openscad
Examples
Rectangle
Ironically, to create a rectangle you use cube():
cube([100, 50, 25]);
Hollow cylendar
Make a hollow cylendar that is 200mm high and 100mm wide with 1mm walls:
difference() {
cylinder(d=100, h=200, $fn = 100);
`cylinder(d=99, h=200, $fn = 100);
}
Note that if you set $fn = 4 you get a hollow rectangle.