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The original IBM PC keyboard didn't have separate cursor keys; the numeric keypad doubled as such. It wasn't long, however, until a new keyboard was introduced that did have separate cursor keys (so effectively two sets when num lock was turned off), and that was the layout that desktop keyboards used thereafter.

Why the change? Having just the cursor keys on the numeric keypad makes the keyboard take up less space as well as cost less, so on the face of it would seem preferable; presumably there was demand for the new layout. Was it from people wanting to use cursor navigation while entering numeric data? In that case, it would seem much better to let the left hand letter keys double as a second set of cursor keys, so that one could use both hands at the same time. Was there some advantage to the separate cursor keys that I'm overlooking?

I think this keyboard style was introduced by the IBM RT PC in 1986 (I'm ignoring other non PC class machines and workstations) For a PC the first use I saw was on a Compaq Deskpro 386 the keyboard is explicitly called a 101 key style IBM RT PC in this review.

https://archive.org/details/byte-magazine-1987-02/page/n247?q=deskpro+386

I don't remember seeing any great clamour for extra arrow keys, but I'm guessing folks who did a lot of spreadsheets were greatful.

I remember a conversation with an IBM engineer back in the 1980s, who implied there was an internal fight over this between IBM's engineering and marketing departments.

The engineers wanted the PC to have the same keyboard as the popular 3270 mainframe terminal, for easy migration of users and software in a business environment, and in fact IBM did produce the "IBM 3270 personal computer" with this keyboard.

However the marketing guys wanted something smaller and cheaper than the 122-key 3270 design with independent cursor keys and numeric keypad, plus 24 function keys above the main keyboard and a further block of 10 special-purpose "function keys" to the left of it. The original 3270 keyboards were heavy as well as big, since they had sheet metal cases rather than plastic.

The original IBM PC keyboard repurposed the 10 left-hand keys as general purpose function keys, deleted the original 24 function keys, and compressed the layout of the right hand side to save space. The PCjr keyboard went even further and reduced the total number of keys down to 62.

These PC keyboards were not popular with users who were accustomed to mainframe terminals, and eventually the 101 or 102 key "enhanced keyboard" design emerged as a compromise, and remained as basis for the current PC "standard keyboard."

See http://www.quadibloc.com/comp/kyb03.htm for layouts of the various keyboards referred to.

I suppose the idea of mapping the cursor keys onto part of the main keyboard with a separate "function key" to toggle that behaviour simply didn't occur to anyone at IBM at the time, coming from a "big keyboard" background. Even now, laptop users who want to use keyboard-intensive applications usually buy an add-on keypad.