Why chairs have four legs ?

Commonly in our day to day life we see many chairs, tables, cupboards etc. All of them have one thing in common, they stand on four legs. Why do most of the time they have four legs and not three or five, six or may be more ? 

What is so special about four legs ? Is there any science behind it or is it just some age old custom that has not changed in years ? Let's find out! 

WHY CHAIRS DO NOT HAVE THREE LEGS INSTEAD ?

 Let's at first see why chirs and most table have four legs and not three. Well, this has a very proper reasoning and that is Centre of gravity! Yes and besides that aesthetic appeal also plays a very vital role (somewhat).

 1. Aesthetic appeal - Humans are known very well for their likeliness for symmetrical objects and this tendency to judge based on symmetry is not only limited to chairs or tables but also fellow humans. You often see that beauty pageants who succeeded in gaining the first position most if not every time have symmetrical face, almost perfect symmetry! Well, even if this is not the only deciding factor but it surely is one.

 Four legs gives more symmetry than three legs and so yes it's naturally more preferable over three legs to implement it in the design. Now let's go for the more scientific approach.

 2. Unstable Center of gravity - This is the most important reason as to why people resorted to four legged chairs if not already. Look at the diagrams given below it shows a chair with three legs. In the figure Athe person is sitting straight and not leaning on his back. The second figure B shows the person leaning on his back on the chair.