ISO/IEC 9995-4
ISO/IEC 9995-4 specifies the layout of the numeric section of a keyboard, if such one exists. It is subdivided into the function zone (shown with grey background in the left figure) and the numeric zone (shown with white background). The leftmost key in the lowest row may span to the left, occupying the position shown with yellow background.
The decimal digits 1…9 may be arranged in an “1-2-3 layout” (shown in green, named according to he keys in the D row), or in a ”7-8-9 layout” (shown in blue).
On a keyboard used for telematic functions, the symbols “” and “#” represent the initiator and the terminator. On a keyboard used for office purposes, the key denoted by “#” shall show the decimal separator (usually a dot or a comma, dependent on the user language). On such keyboards, the key position marked by the asterisk may be an extension of the “0” key, or a “double zero” key. Accordingly, if the yellow key position is used, it may be also an extension of the “0” key, a “double zero” key, or a “triple zero” key.
The keys in the function zone may be associated to the arithmetic function they denote, or to the arithmetic characters, dependent of the software which is used. If an additional tabulator key or another special key is used, it shall be located at the position C54 (according to the reference grid specified in ISO/IEC 9995-1), thus occupying the lower half of the place shown for the “=” key.
It is to be noted that numeric sections on usual keyboards for personal computers do not comply to the standard, as they usually have a Num lock key in the upper left corner where the standard requires the “+” key, and therefore also show a different arrangement of the other arithmetic keys (usually lacking an “=” key; see the second picture). By this, the standard in its present form (ISO/IEC 9995-4:2009) can be considered outdated.
Read more about this topic: ISO/IEC 9995