These notes spell out the ideas behind the throwaway remark about chording keyboards and eight-bit bytes in the essay "Seven Great Blunders of the Computing World" (Computer, 2000 July, p.111).

Direct Keying

The idea of a chording keyboard for eight-bit bytes springs from several ideas.

• Such bytes are commonly represented, and sometimes remembered, as a pair of hexadecimal digits.
• The four bits of a hexadecimal digit map into the four fingers (excluding the thumb for the moment), let's say index finger low order, little finger high order.
• The chording keyboard has one key for each finger, and thus for each bit of the two hexadecimal digits.
• A value of 1 in a bit position is represented by the depression of the key at that position.
• To get around the problem of keying in the NULL character, which being all zero bits would be undetectable, a key for the thumb is used to invert the bits of the fingers.  This allows the NULL character to be keyed in by pressing down with all fingers and thumbs.

Hexadecimal Digits

The inverting with the thumb allows two key encoding of all hexadecimal digits except the zero.  This feature could help in reducing fatigue from keying.  These two-key encodings are shown in the following table.

Hex	Binary	Fingers		Hex	Binary	Fingers		Hex	Binary	Fingers		Clews
1	0001	I		6	0110	RM		B	1011	TR		~	T	Thumb
2	0010	M		7	0111	TL		C	1100	LR		1	I	Index
3	0011	MI		8	1000	L		D	1101	TM		2	M	Middle
4	0100	R		9	1001	IL		E	1110	TI		4	R	Ring
5	0101	RI		A	1010	LM		F	1111	T		8	L	Little

Each hexadecimal digit except 0 has two key encodings, one with the thumb down and one with it up.  The exception is because, although theoretically 0 can be encoded with no keys depressed, that encoding cannot be detected in itself as there is neither a make nor a break signal from the key switches.  Therefore the 0 must be encoded by depressing all five keys.

Eight-bit Characters

There are two standard character sets in common use for the Latin alphabets, the 7-bit ASCII character set based on early telegraph codes and used on smaller computers, and the 8-bit EBCDIC character set based on the 6-bit BCD encoding of the computers of the 1950s and on the most popular encoding for punched cards.

There has been a great variety of 8-bit ASCII extensions, and so both encoding are frequently represented as a double hexadecimal digit.  The encodings for the two Latin alphabets is given in the following table.

	ASCII	EBCDIC		ASCII	EBCDIC		ASCII	EBCDIC
A a	41 61	C1 81	J j	4A 6A	D1 91	S s	53 73	E2 A2
B b	42 62	C2 82	K k	4B 6B	D2 92	T t	54 74	E3 A3
C c	43 63	C3 83	L l	4C 6C	D3 93	U u	55 75	E4 A4
D d	44 64	C4 84	M m	4D 6D	D4 94	V v	56 76	E5 A5
E e	45 65	C5 85	N n	4E 6E	D5 95	W w	57 77	E6 A6
F f	46 66	C6 86	O o	4F 6F	D6 96	X x	58 78	E7 A7
G g	47 67	C7 87	P p	50 70	D7 97	Y y	59 79	E8 A8
H h	48 68	C8 88	Q q	51 71	D8 98	Z z	5A 7A	E9 A9
I i	49 69	C9 89	R j	52 72	D9 99		20	40

The last entry in the table is for the blank character.  The peculiar nature of both these encodings in their full form is irrelevant here.  Suffice it to say that neither has found reason to provide an encoding for the basic arithmetic symbols ÷ and ×, to the eternal shame of the industry.

What it is relevant to note here is that, with a chording keyboard allotting the high order hexadecimal digit to the left hand, that left hand functions mainly as a shifting hand, with the variation during a keying sequence going primarily to the right hand.  Of course, all such keyboards should provide a hand-swap option to suit left-handers.

Difference Keying

xxx

the table of the EBCDIC character set the table of the ASCII character set