Spiral stairs wind around a newel - the central pole - and typically have a handrail on the outer side only. However, more elaborate spiral staircases can be designed, including an additional rail on the inner; mirroring the tight curvature of the central pole.
Squared spiral staircases are common place, with the structure assuming the full space of a square stairwell. This expands the stairs and railing to the size of the square, resulting in unequal steps; larger where they extend into a corner of the square, but making full use of available space.
A pure spiral staircase assumes just a circular stairwell, with the steps and handrail being equal and positioned screw-symmetrically. A tight spiral stair with a central pole provides an excellent efficient use of space - in terms of floor area - which makes it work particularly well within smaller projects.
Spiral staircases can have the disadvantage of often being steep; if they are tight or are otherwise not supported by a central column. The wider the spiral, the greater the number of steps that can be accommodated per spiral. If the spiral is large in diameter - providing an exceptional central support column that is both large and strong - and is combined with a special handrail, that helps distributes the load, each step may be longer. This allows the rise between each step may be smaller; equal to that of regular steps.
In order to maintain headroom, some spiral stairs have very high rises to support a very short diameter. These are typically cases where the stairwell must be a small diameter by design, must not have any centralised support or may not have any outer perimeter support.