Hierarchy of Decimal Numbers
Number

Name

How many

0  zero  
1  one  
2  two  
3  three  
4  four  
5  five  
6  six  
7  seven  
8  eight  
9  nine  
10  ten  
20  twenty  two tens 
30  thirty  three tens 
40  forty  four tens 
50  fifty  five tens 
60  sixty  six tens 
70  seventy  seven tens 
80  eighty  eight tens 
90  ninety  nine tens 
Number  Name  How Many 
100  one hundred  ten tens 
1,000  one thousand  ten hundreds 
10,000  ten thousand  ten thousands 
100,000  one hundred thousand  one hundred thousands 
1,000,000  one million  one thousand thousands 
Some people use a comma to mark every 3 digits. It just keeps track of the digits and makes the numbers easier to read.
Beyond a million, the names of the numbers differ depending where you live. The places are grouped by thousands in America and France, by the millions in Great Britain and Germany.
Name  AmericanFrench  EnglishGerman 
million  1,000,000  1,000,000 
billion  1,000,000,000 (a thousand millions)  1,000,000,000,000 (a million millions) 
trillion  1 with 12 zeros  1 with 18 zeros 
quadrillion  1 with 15 zeros  1 with 24 zeros 
quintillion  1 with 18 zeros  1 with 30 zeros 
sextillion  1 with 21 zeros  1 with 36 zeros 
septillion  1 with 24 zeros  1 with 42 zeros 
octillion  1 with 27 zeros  1 with 48 zeros 
googol 
1 with 100 zeros
 
googolplex 
1 with a googol of zeros

FractionsDigits to the right of the decimal point represent the fractional part of the decimal number. Each place value has a value that is one tenth the value to the immediate left of it.
Number  Name  Fraction 
.1  tenth  1/10 
.01  hundredth  1/100 
.001  thousandth  1/1000 
.0001  ten thousandth  1/10000 
.00001  hundred thousandth  1/100000 
Examples:
0.234 = 234/1000 (said  point 2 3 4, or 234 thousandths, or two hundred thirty four thousandths)
4.83 = 4 83/100 (said  4 point 8 3, or 4 and 83 hundredths)


I=1  (I with a bar is not used)  
V=5  _ V=5,000  
X=10  _ X=10,000  
L=50  _ L=50,000  
C=100  _ C = 100 000  
D=500  _ D=500,000  
M=1,000  _ M=1,000,000 
1 = I 2 = II 3 = III 4 = IV 5 = V 6 = VI 7 = VII 8 = VIII 9 = IX 10 = X  11 = XI 12 = XII 13 = XIII 14 = XIV 15 = XV 16 = XVI 17 = XVII 18 = XVIII 19 = XIX 20 = XX 21 = XXI  25 = XXV 30 = XXX 40 = XL 49 = XLIX 50 = L 51 = LI 60 = LX 70 = LXX 80 = LXXX 90 = XC 99 = XCIX 
There is no zero in the roman numeral system.
The numbers are built starting from the largest number on the left, and adding smaller numbers to the right. All the numerals are then added together.
The exception is the subtracted numerals, if a numeral is before a larger numeral, you subtract the first numeral from the second. That is, IX is 10  1= 9.
This only works for one small numeral before one larger numeral  for example, IIX is not 8, it is not a recognized roman numeral.
There is no place value in this system  the number III is 3, not 111.
Number Base Systems
Decimal(10)

Binary(2)

Ternary(3)

Octal(8)

Hexadecimal(16)

0

0

0

0

0

1

1

1

1

1

2

10

2

2

2

3

11

10

3

3

4

100

11

4

4

5

101

12

5

5

6

110

20

6

6

7

111

21

7

7

8

1000

22

10

8

9

1001

100

11

9

10

1010

101

12

A

11

1011

102

13

B

12

1100

110

14

C

13

1101

111

15

D

14

1110

112

16

E

15

1111

120

17

F

16

10000

121

20

10

17

10001

122

21

11

18

10010

200

22

12

19

10011

201

23

13

20

10100

202

24

14

Each digit can only count up to the value of one less than the base. In hexadecimal, the letters A  F are used to represent the digits 10  15, so they would only use one character.