The bottom part which is four-sided remains underground, the middle part which is eight-sided remains on a pedestal.
The top part, which is actually worshipped, is round.
The height of the round part is one-third of its circumference.
The three parts symbolize :-
Sri. Brahma at the bottom,
Sri. Vishnu in the middle and
Lord Shiva on the top.
The pedestal is provided with a passage for draining away the water that is poured on the top.