ASCII

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Definition

The term ASCII stands for "American Standard Code for Information Interchange" and describes a character set standard for text data and information exchange. Character encodings (also called code pages) define how letters and other text characters, as well as control codes for data transfer, are to be encoded in bits and bytes.

The ASCII code is one of the most important character set standards along with Unicode, ISO-8859-1 and Windows 1252. The code was the dominant standard for web pages for a long time until it was overtaken by the Unicode encoding UTF-8 in 2007. Nevertheless, it is still relevant today, as there are still areas where only characters contained in the ASCII code are allowed.

History

ASCII code has a long history. This history began with telegraphy and Morse code as well as the 5-bit Murray code developed by the New Zealand inventor Donald Murray between 1901 and 1932. The first version of the ASCII code was released in 1963 by ASA, the American Standards Association. ASA was a precursor of the American National Standards Institute (ANSI). In 1968, the version of the character set that is still valid today was published.

The 7-bit ASCII character set

Because of its history, the original ASCII code uses only seven bits of a common 8-bit byte and can encode a maximum of 128 different characters. The code contains upper and lower case letters of the English alphabet, the most important punctuation marks, mathematical symbols, and 33 control codes for data transfer and text formatting.

The following character groups are included:

  • 0-32 and 127: control codes for data transfer as well as spaces, tabs, and line breaks
  • 48-57: digits
  • 65-90: capital letters
  • 97-122: lower case letters
  • 33- 7, 58-64, 91-96 and 123-126: punctuation marks, mathematical symbols, brackets, and other characters

Country-specific special characters such as umlauts and accents are not included in ASCII code.

Although the arrangement may seem chaotic and arbitrary, it results from thorough planning and consideration. The letters are positioned in such a way that upper and lower case versions of a letter only differ by a single bit. Numbers, spaces, and some other symbols are deliberately placed in front of the letters to simplify sorting. In addition, many non-alphanumeric symbols are located on positions similar to typewriter arrangements.

Extended character sets: ISO-8859

Since the actual ASCII code only contains the English alphabet, many region-specific extensions have been developed. In this context, the character sets Windows-1252 and ISO-8859-1 have gained particular importance. Both are 8-bit extensions of the original standard and contain many special regional characters. Due to historical developments, both standards are often referred to as ANSI character sets. Strictly speaking, however, this is not correct because ANSI has never officially standardized these character sets.

However, even with 8 bits, only 256 characters are available and therefore not all languages can be covered. Therefore, ISO-8859 has been developed as a collection of different character sets for different languages and regions. For example, ISO-8859-7 contains Latin and Greek alphabets, while ISO-8859-4 covers the special characters of Scandinavian and Baltic languages. ISO-8859-1 contains Western European alphabets and is almost identical to Windows 1252.

For both Windows 1252 and ISO 8859-1, the first 128 characters are identical to ASCII code. From position 128, code-specific special characters follow, whereby the numbers 128 to 159 are undefined in the ISO-8859 standard. Starting with number 160, the special characters of the different languages and regions are contained.

ASCII, Unicode, and UTF-8

Although the ISO-8859 standards cover many languages, not all languages are included. In addition, the different character sets caused a considerable amount of confusion because they are not compatible with each other. As early as 1988, the first plans for a uniform Unicode character set were drawn up, the first version of which was released in 1991.

Unicode enables the display of over a million characters and gradually replaces all other character sets. The Unicode encoding UTF-8, which now is the predominant text format on the World Wide Web, is particularly important. UTF-8 has the big advantage that it is ASCII compatible since the first 128 characters are identical.

Structure of ASCII and ISO tables

Usually, lists or tables are used to display the character sets in order to make the characters and their numerical values easy to find. These lists specify the characters and their decimal, hexadecimal, octal, and/or binary values.

Many tables are hexadecimal and separate the codes into the first and second half bytes. For example, the large H in the ASCII table is found in the 4th row of the 8th column, resulting in the hexadecimal notation 0x48. The carriage return CR has the code 0x0D because it is in line 0 and column D. 0x is a common prefix to refer to the hexadecimal notation.

ASCII Table

In the following, you can see the ASCII table with codes in decimal, hexadecimal and octal notation:

char. decimal hexadec. octal
NUL 0 0x00 000
SOH 1 0x01 001
STX 2 0x02 002
ETX 3 0x03 003
EOT 4 0x04 004
ENQ 5 0x05 005
ACK 6 0x06 006
BEL 7 0x07 007
BS 8 0x08 010
HT 9 0x09 011
LF 10 0x0A 012
VT 11 0x0B 013
FF 12 0x0C 014
CR 13 0x0D 015
SO 14 0x0E 016
SI 15 0x0F 017
DLE 16 0x10 020
DC1 17 0x11 021
DC2 18 0x12 022
DC3 19 0x13 023
DC4 20 0x14 024
NAK 21 0x15 025
SYN 22 0x16 026
ETB 23 0x17 027
CAN 24 0x18 030
EM 25 0x19 031
SUB 26 0x1A 032
ESC 27 0x1B 033
FS 28 0x1C 034
GS 29 0x1D 035
RS 30 0x1E 036
US 31 0x1F 037
char. decimal hexadec. octal
SP 32 0x20 040
 ! 33 0x21 041
" 34 0x22 042
# 35 0x23 043
$ 36 0x24 044
 % 37 0x25 045
& 38 0x26 046
' 39 0x27 047
( 40 0x28 050
) 41 0x29 051
* 42 0x2A 052
+ 43 0x2B 053
, 44 0x2C 054
- 45 0x2D 055
. 46 0x2E 056
/ 47 0x2F 057
0 48 0x30 060
1 49 0x31 061
2 50 0x32 062
3 51 0x33 063
4 52 0x34 064
5 53 0x35 065
6 54 0x36 066
7 55 0x37 067
8 56 0x38 070
9 57 0x39 071
 : 58 0x3A 072
 ; 59 0x3B 073
< 60 0x3C 074
= 61 0x3D 075
> 62 0x3E 076
 ? 63 0x3F 077
char. decimal hexadec. octal
@ 64 0x40 100
A 65 0x41 101
B 66 0x42 102
C 67 0x43 103
D 68 0x44 104
E 69 0x45 105
F 70 0x46 106
G 71 0x47 107
H 72 0x48 110
I 73 0x49 111
J 74 0x4A 112
K 75 0x4B 113
L 76 0x4C 114
M 77 0x4D 115
N 78 0x4E 116
O 79 0x4F 117
P 80 0x50 120
Q 81 0x51 121
R 82 0x52 122
S 83 0x53 123
T 84 0x54 124
U 85 0x55 125
V 86 0x56 126
W 87 0x57 127
X 88 0x58 130
Y 89 0x59 131
Z 90 0x5A 132
[ 91 0x5B 133
/ 92 0x5C 134
] 93 0x5D 135
^ 94 0x5E 136
_ 95 0x5F 137
char. decimal hexadec. octal
` 96 0x60 140
a 97 0x61 141
b 98 0x62 142
c 99 0x63 143
d 100 0x64 144
e 101 0x65 145
f 102 0x66 146
g 103 0x67 147
h 104 0x68 150
i 105 0x69 151
j 106 0x6A 152
k 107 0x6B 153
l 108 0x6C 154
m 109 0x6D 155
n 110 0x6E 156
o 111 0x6F 157
p 112 0x70 160
q 113 0x71 161
r 114 0x72 162
s 115 0x73 163
t 116 0x74 164
u 117 0x75 165
v 118 0x76 166
w 119 0x77 167
x 120 0x78 170
y 121 0x79 171
z 122 0x7A 172
{ 123 0x7B 173
124 0x7C 174
} 125 0x7D 175
~ 126 0x7E 176
DEL 127 0x7F 177

ASCII Code, Unicode, and SEO

Although ASCII and ISO-8859 were the predominant text character standards for a long time, they are considered obsolete on the web today. The official standardization organization W3C (World Wide Web Consortium) recommends the exclusive use of UTF-8 as character encoding for all websites.

In addition to the actual texts on a web page, Unicode can also be used in meta descriptions. Unicode characters such as hooks, hearts, stars, envelopes or currency symbols can trigger unconscious impulses in the reader. For example, checkmarks and hearts generate approval, while envelopes and telephone symbols encourage contact. While this has no direct impact on search engine rankings, it increases click-through rates and leads to more visitors and customers.

In SEO-relevant keywords and keyword phrases, however, some restraint is advisable. Country-specific letters like umlauts and accents are no problem. However, unusual special characters, separating symbols, emoticons and pictograms can make keyword recognition impossible.