A cylinder is a solid geometric shape that always has two ends lying parallel to each other and connected by a single side with a circular cross-section. The ends of a cylinder, which are called its bases, are usually circular but can be elliptical. A cylinder with circular bases is known as a right cylinder because its bases lie perpendicular to the shape's axis at a 90-degree right angle.
Cylinders, both naturally occurring and artificial, are abundant in the world. Circles are naturally formed by many processes, and joining them to form a cylinder is no more complicated than rolling up a rectangle until its edges meet.
Because of their circular cross-section, cylinders tend to be strong and resistant to lateral compression. This has made the shape attractive to engineers designing pipes and space scientists designing modules for space travel. Rocket tubes are also cylindrical, as the large internal volume and smooth side of a cylinder help to maximize payload while keeping drag to a minimum. The single side of a cylinder also gives the shape considerable strength against vertical compression, as it resists downward force by supporting its top base at every point. This is why ancient stone buildings often mount cylindrical columns to support their roofs.