Background

In mechanical typewriters, the shift key functions by mechanically shifting some component so an alternate row of characters on typebars hits the paper. In an electronic system, by contrast, there is no necessary connection between the code points of unshifted and shifted values, though implementation is simpler if the code points of unshifted and shifted keys are related, most simply by a single bit differing. In electromechanical systems, this makes a significant difference in ease of implementation, as shifting must be accomplished by some physical linkage.

For this reason, among others (such as ease of collation), the ASCII standard strove to organize the code points so that shifting could be implemented by simply toggling a bit. This is most conspicuous in uppercase and lowercase characters: uppercase characters are in columns 4 (100) and 5 (101), while the corresponding lowercase characters are in columns 6 (110) and 7 (111), requiring only toggling the 6th bit (2nd high bit) to switch case; as there are only 26 letters, the remaining 6 points in each column were occupied by symbols or, in one case, a control character (DEL, in 127).

This is also present, but less precisely, in the organization of digits and symbols in columns 2 (010) and 3 (011) – this discrepancy is the source of bit-paired layouts. Ideally the characters would have been ordered so that unshifted and shifted values of a typewriter key were in adjacent columns, allowing shifting to be implemented by toggling the 5th bit (1st high bit). Due to other concerns, this correspondence is inexact: for example, SP (Space) and 0 (zero) both have low bits 00000 (to ease collation for space and conversion to/from binary-coded decimal for 0), preventing 0 from lining up with ) (right parenthesis), its conventional value, and thus instead () corresponded to 89, instead of 90 as on typewriters. Further, while digits were placed in column 3, the characters ,-./ (conventionally unshifted) were placed in column 2, to ease collation, due to being used as separators, and the characters ;: (conventionally paired) were both placed in column 3. Other symbols also did not line up with their conventional digit pair, as detailed below.

As a result, implementing an electromechanical keyboard that produced an ASCII encoding but had conventional typewriter key mappings would require significant complexity due to key-specific shift mechanisms for digits and symbol keys. This could be avoided by changing the key mappings to correspond to the ASCII table, which was notably done in the Teletype Model 33 (1963). Later keyboards continued to use this mapping, which was formalized in the American Standards Association X4.14-1971 standard and the European Computer Manufacturers' Association ECMA-23 standard, where it is referred to as logical bit pairing, and contrasted with typewriter pairing. In everyday usage these were referred to as bit-paired and typewriter-paired keyboards.

Mapping

The most conspicuous feature of a bit-paired keyboard are the shifted values of the digits, which are (only 9 values, 0 being unpaired):

!"#$%&'()

…instead of the conventional mechanical typewriter-paired values of:

!"#$%_&'()

…and the electric typewriter-paired values of:

!@#$%^&*()

The shifted values of digits in a bit-paired keyboard thus differ from mechanical typewriters only in omitting _ and shifting the remaining characters left. Electric typewriters differ in replacing " with @, _ with ^, and ' with *. This is because smaller characters hit the paper with less force, and consolidating smaller characters such as '" and -_ into a pair on a single key avoided needing to adjust the force based on shift state.^[1]

Overall this results in the following comparison between bit-paired keyboards and (electric) typewriter-paired keyboards: they agree in 1, 3, 4, 5, and differ in 2, 6, 7, 8, 9, 0: bit-pairing replaces @ with " (position 2), removes ^ (position 6, shifting later characters left), and replaces * with ' (position 8/7); there is also no shifted value for 0, as this corresponds to space, which is covered by the space bar. This corresponds in the ASCII table to the following columns (displayed as rows here):

high  low bits
bits  0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
 010        !    "    #    $    %    &    '    (    )
 011   0    1    2    3    4    5    6    7    8    9

There are also less conspicuous differences in the symbol keys: bit-paired and typewriter-paired keyboards agree on ^<
_,^>
_.^?
_/ (rows 12, 14, and 15) but bit-paired keyboards have ^*
_:⁺
_;⁼
_- (rows 10, 11, and 13) instead of ^:
_;⁺
₌^_
_- and are missing the '" key, as these are shifted values of digits.

This is the complete set of symbols on the Model 33; it is notably missing @`[{\|]}^~_ though these are present on other bit-paired keyboards. On both bit-paired and typewriter-paired keyboards, <sup>{</sup>
<sub>[</sub><sup>|</sup>
<sub>\</sub><sup>}</sup>
<sub>]</sub> are paired (rows 11, 12, and 13), but on bit-paired keyboards <sup>`</sup>
_@^~
_^ are paired and _ is an unpaired key, while on typewriter-paired keyboards @ and ^ are shifted digits, while ^~
_` are paired and _ is paired with -.