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 google of zeros

Fractions
Digits 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)
SI Prefixes
Number 
Prefix 
Symbol 
10 ^{1} 
deka 
da 
10 ^{2} 
hecto 
h 
10 ^{3} 
kilo 
k 
10 ^{6} 
mega 
M 
10 ^{9} 
giga 
G 
10 ^{12} 
tera 
T 
10 ^{15} 
peta 
P 
10 ^{18} 
exa 
E 
10 ^{21} 
zeta 
Z 
10 ^{24} 
yotta 
Y 

Number 
Prefix 
Symbol 
10 ^{1} 
deci 
d 
10 ^{2} 
centi 
c 
10 ^{3} 
milli 
m 
10 ^{6} 
micro 
u (greek mu) 
10 ^{9} 
nano 
n 
10 ^{12} 
pico 
p 
10 ^{15} 
femto 
f 
10 ^{18} 
atto 
a 
10 ^{21} 
zepto 
z 
10 ^{24} 
yocto 
y 

Roman Numerals
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 
Examples:
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.