The barley twist is one of the few turned forms that cannot be produced by turning alone. A lathe gives you the tapering profile; the spiral has to be laid out along the length and then cut and filed by hand, following a line drawn around the column. Every twist on a set of four has to keep the same pitch, or the columns disagree with each other at a glance.
It became popular in the seventeenth century and stayed because of what it does to light: a spiral has no flat face, so it is lit on one side and shadowed on the other continuously up its height, which makes a column look alive rather than static.
Setting four of them under an oval rather than a rectangle is the harder arrangement. The canopy frame is a continuous curve, so each column meets it at a slightly different angle, and the joints have to be cut individually. Our master Indian artisans build the oval first and fit the columns to it, never the other way round - the only way to keep the canopy true.