Number system is a system of representing the number in different forms. Such as whole numbers,fractions, decimals etc.
While reading numbers it is always easy to use words instead of reading individual digits.

For example : Instead of reading 3247 as Three, Two, Four, Seven, it is easy to read it as “Three thousand,two hundred and forty seven”.
In this section, we shall discuss various methods of expressing a number in words.
There are two commonly used methods of numeration.

Indian System : In this system a number is spilt up into groups or periods. In Indian system of numeration we use ones,tens,hundreds,thousands and then lakhs and crores.the first comma comes after hundreds place(three digits from the right).the second comma comes two digits later(five digits from the right) the third comma comes after another two digits and so on.
See the following table.

Periods

Arabs

Crore

Lac

Thousands

Ones

Places

10 A| 1 A

10 C | 1 C

10 L| 1 L

10 th | 1 th

100 | 10 | 1

Example : 1) Write 123456789 according to Indian system. Solution : 12,34,56,789 -> Twelve Crore,thirty four lac,fifty six thousand, seven hundred and eighty nine.
Example : 2) Insert the commas in 1112323 and write it in words. Solution: 11,12, 323 --> Eleven lac,twelve thousand, three hundred and twenty three.
Example : 3) Write " Eight crore two lac seventy-four thousand forty four" in numerals. Solution:8,02,74,044