How the idea of 30 ml, 60 ml and 90 ml drinks get started? Where did the idea of pouring 30 ml, 60 ml or 90 ml in drinks begin. Is there a theory or scientific reason behind it? Have gone through this post: Peg (unit) from Wikipedia, but it does not help me.


  A peg is a unit of volume for measuring liquor in India and Nepal. The terms "large peg" and "small peg" are used, equal to 60 mL and 30 mL, respectively, with "peg" simply referring to a small peg. In India liquor's alcohol content is fixed at 42.8% ABV, it follows that a peg of liquor contains 25.68 mL of pure alcohol, or 20.26 g.


And then there is this also: Patiala peg (Wikipedia).


  The Patiala peg is a measure of liquor popular in Indian Punjab. It is a volume roughly equivalent to 120 ml, though the rough and ready measure is the amount of liquor needed to fill a glass equal to the height between the index and little fingers when they are held parallel to one another.[1] Even major liquor companies have started selling their products in the single drink packaging of 90 ml &amp; 120 ml bottles in India. The name originates from the city of Patiala, which was once a state known for the extravagant ways of its royalty and extraordinary height of its Sikh soldiers.


Did it originate in India ?
The British used the Imperial system of measures from 1824 and India was part of the British Empire until 1947. 1 British Imperial fluid ounce is equal to 28.41 ml, which is close to 30 ml. Maybe this practice evolved from the specification that 1 peg equaled 1 imp. oz. and was rounded up to 30-ml after metrification. 

Standard Indian-made liquor is packaged in 90-, 120-, 180-, 360-, and 750-ml bottles to make it easier to divide equally into pegs.