Once people have had for the last number. Once this number was very small, for example, one hundred. And even earlier - even forty. Currently, the numbers have become so large that for ease of reading typed classes. The right three separate numbers-1st class, then another three or 2nd class, etc. 1st class gives the number of units, the 2nd - thousand, 3 - million. Unit 4-th class is called billion or billion. Unit 5 th, 6 th, 7 th, 8 th... classes are called trillion, quadrillion, quintillion, sextillion, septillion... So, we can say that 1000 is a unit of the 2nd class, and 15 trillion fifteen units 5. Each unit of the following class is a thousand times more than the previous. Thus, with the introduction of classes you can hardly count to the end.

In the decimal system is the smallest number is 1 (10^{0}), and then go: 10 - ten, 10^{2} hundred, 10^{3} - one thousand, 10^{6} - million, 10^{9} - billion (billion), 10^{12} - trillion, 10^{15} - quadrillion, 10^{18} - quintillion, 10^{21} - sextillion, 10^{24} - septillion, 10^{27} - octillion, 10^{30} - nonillion, 10^{33} - decillion...