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Almost every book on programming will give you a list of ASCII characters, and almost none of them will bother giving you any clue as to why. Hopefully, a little bit of colour helps clear this up.

These days, the logical unit for a small amount of information is the byte: each byte stores 8 = 2³ bits and is capable of storing a number between 0 and 255. In the 1960s, when space was at much more of a premium, it was recognized that most communications of the English language could be performed with 128 characters, or seven bits. Thus, the ASCII characters occupy the first 128 characters. The remaining 128 characters vary, but often encode special symbols and accented letters.

We will break the ASCII letters into four groups:

• 0-31 Control characters
• 32-63 Printable characters (with embedded decimal digits)
• 64-95 Printable characters (with embedded capital letters)
• 96-127 Printable characters (with embedded lower-case letters)

Special room is made for each of three sets: the decimal digits 0-9, the capital letters A-Z, and the lower-case letters a-z. All other symbols fill in the gaps around these. The characters on the keyboard today are a direct consequence of those characters which can be printed using ASCII.

Each of these examined in the next four sections followed by a summary.

Control Characters (0-31)

The first 32 ASCII characters are control characters, that is, they are used to control printing devices, including such instructions such as, for example, to indicate the end of a line, to signal a tabl, or an instruction to ring a bell.

If the first three bits are 000, this indicates that ASCII character is control character.

Note: There is one more control character: delete is 0111 1111 or 7F.

Binary	Dec	Hex	Glyph
0000 0000	0	00	Null character (NUL)
0000 0001	1	01	Start of Header (SOH)
0000 0010	2	02	Start of Text (STX)
0000 0011	3	03	End of Text (ETX)
0000 0100	4	04	End of Transmission (EOT)
0000 0101	5	05	Enquiry (ENQ)
0000 0110	6	06	Acknowledgment (ACK)
0000 0111	7	07	Bell (BEL)
0000 1000	8	08	Backspace (BS)
0000 1001	9	09	Horizontal Tab (HT)
0000 1010	10	0ALine feed (LF)
0000 1011	11	0BVertical Tab (VT)
0000 1100	12	0C	Form feed (FF)
0000 1101	13	0D	Carriage return (CR)
0000 1110	14	0E	Shift Out (SO)
0000 1111	15	0F	Shift In (SI)
0001 0000	16	10	Data Link Escape (DLE)
0001 0001	17	11	Device Control 1 (DC1)
0001 0010	18	12	Device Control 2 (DC2)
0001 0011	19	13	Device Control 3 (DC3)
0001 0100	20	14	Device Control 4 (DC4)
0001 0101	21	15	Negative Acknowledgement (NAK)
0001 0110	22	16	Synchronous Idle (SYN)
0001 0111	23	17	End of Transmission Block (ETB)
0001 1000	24	18	Cancel (CAN)
0001 1001	25	19	End of Medium (EM)
0001 1010	26	1A	Substitute (SUB)
0001 1011	27	1B	Escape (ESC)
0001 1100	28	1C	File Separator (FS)
0001 1101	29	1D	Group Separator (GS)
0001 1110	30	1E	Record Separator (RS)
0001 1111	31	1F	Unit Separator (US)

Printable Characters I (32-63)

All but one of the remaining characters are printable. Embedded within the first 32 of these are the decimal digits 0 through 9. These decimal digits begin with the four bits 0011 and if the character is a decimal digit (0011 0000 through 0011 1001), it can be converted to an integer by replacing the leading 0011 with 0000.

Binary	Dec	Hex	Glyph
0010 0000	32	20	(blank)
0010 0001	33	21	!
0010 0010	34	22	"
0010 0011	35	23	#
0010 0100	36	24	$
0010 0101	37	25	%
0010 0110	38	26	&
0010 0111	39	27	'
0010 1000	40	28	(
0010 1001	41	29	)
0010 1010	42	2A	*
0010 1011	43	2B	+
0010 1100	44	2C	,
0010 1101	45	2D	-
0010 1110	46	2E	.
0010 1111	47	2F	/
0011 0000	48	30	0
0011 0001	49	31	1
0011 0010	50	32	2
0011 0011	51	33	3
0011 0100	52	34	4
0011 0101	53	35	5
0011 0110	54	36	6
0011 0111	55	37	7
0011 1000	56	38	8
0011 1001	57	39	9
0011 1010	58	3A	:
0011 1011	59	3B	;
0011 1100	60	3C	<
0011 1101	61	3D	=
0011 1110	62	3E	>
0011 1111	63	3F	?

Printable Characters II (64-95)

Embedded within the next 32 characters embed the capital letters A through Z. These letters begin with the three bits 010 and if the character is a letter (0100 0001 through 0101 1010), it can be converted into the integer representing the position of that letter in the alphabet by replacing the out the leading 010 with 000.

For example, you will note that W is encoded using 10111₂ = 23₁₀ and is the 23rd letter in the alphabet.

Binary	Dec	Hex	Glyph
0100 0000	64	40	@
0100 0001	65	41	A
0100 0010	66	42	B
0100 0011	67	43	C
0100 0100	68	44	D
0100 0101	69	45	E
0100 0110	70	46	F
0100 0111	71	47	G
0100 1000	72	48	H
0100 1001	73	49	I
0100 1010	74	4A	J
0100 1011	75	4B	K
0100 1100	76	4C	L
0100 1101	77	4D	M
0100 1110	78	4E	N
0100 1111	79	4F	O
0101 0000	80	50	P
0101 0001	81	51	Q
0101 0010	82	52	R
0101 0011	83	53	S
0101 0100	84	54	T
0101 0101	85	55	U
0101 0110	86	56	V
0101 0111	87	57	W
0101 1000	88	58	X
0101 1001	89	59	Y
0101 1010	90	5A	Z
0101 1011	91	5B	[
0101 1100	92	5C	\
0101 1101	93	5D	]
0101 1110	94	5E	^
0101 1111	95	5F	_

Printable Characters III (96-127)

Embedded within the last 32 characters embed the lower-case letters a through z. These letters begin with the three bits 011 and if the character is a letter (0110 0001 through 0111 1010), it can be converted into the integer representing the position of that letter in the alphabet by replacing the out the leading 011 with 000.

For example, you will note that w is coded using 10111₂ = 23₁₀ and is the 23rd letter in the alphabet.

Binary	Dec	Hex	Glyph
0110 0000	96	60	`
0110 0001	97	61	a
0110 0010	98	62	b
0110 0011	99	63	c
0110 0100	100	64	d
0110 0101	101	65	e
0110 0110	102	66	f
0110 0111	103	67	g
0110 1000	104	68	h
0110 1001	105	69	i
0110 1010	106	6A	j
0110 1011	107	6B	k
0110 1100	108	6C	l
0110 1101	109	6D	m
0110 1110	110	6E	n
0110 1111	111	6F	o
0111 0000	112	70	p
0111 0001	113	71	q
0111 0010	114	72	r
0111 0011	115	73	s
0111 0100	116	74	t
0111 0101	117	75	u
0111 0110	118	76	v
0111 0111	119	77	w
0111 1000	120	78	x
0111 1001	121	79	y
0111 1010	122	7A	z
0111 1011	123	7B	{
0111 1100	124	7C	|
0111 1101	125	7D	}
0111 1110	126	7E	~
0111 1111	127	7F	Delete (DEL)

Summary

The general positioning of the various control and printable characters are summarized in Figure 1.

Figure 1. The general layout of ASCII characters.

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