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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?

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    I can't answer the question, but I do want to say that one shouldn't discount the extra cognitive load of having to remember yet another mode that the keyboard can switch between. Even caps lock, while justifiable, causes its share of problems. – Robert Fisher Jan 16 at 16:08
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    I use Mac OS X all the time, which has no concept of Num Lock: the number pad is a number pad, period. (The Num Lock key acts as a "Clear" key instead; I don't think anyone really uses it.) When I try to enter a string of numbers in Windows, I usually stop and wonder why nothing is happening before figuratively smacking myself. (: – SilverWolf Jan 16 at 18:06
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    In early 80's having separate cursor keys was something of a big feature. I remember the Commodore 64 getting criticized about it, and that why the +4 got them, though they were pretty crude. – Ross Ridge Jan 17 at 20:56
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    Have you ever tried to use a numeric keypad for cursor keys? The + shape, as opposed to inverted-T, is not at all comfortable. – R.. Jan 18 at 1:13
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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.

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    Interesting that there was an internal IBM argument. – PeterI Jan 16 at 11:24
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    @PeterI, not really. Once a company gets big enough, it stops functioning as a single coherent entity, and IBM was certainly big enough at that time. – Mark Jan 16 at 20:54
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    Add-on numeric keypads were on sale for desktop PCs in the days of the 84-key keyboard. – grahamj42 Jan 17 at 0:12
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    @Mark, fully agree that once a company gets big enough there will be internal arguments. The size required is two people. – Tero Lahtinen Jan 17 at 14:45
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    What model of 3270 terminal was this 122-key keyboard introduced with? The 3270 series itself appears to have had a different keyboard with no numeric keypad, as detailed at your link, and the DEC VT220 keyboard appears far more similar to the 101-key PC keyboard. – Random832 Jan 17 at 18:29
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I think the answer is Lotus 1-2-3. Working in a spreadsheet, you are going to want to be able to use the numeric keypad for quick numeric entry, and you are going to need to use the cursor keys to move around the spreadsheet. Having to continually hit the Num Lock to switch between "modes" would be a pain.

I have no actual evidence to back this up, other than personal experience, and a sense the timing of Lotus's explosion on the platform coincided with the switch to the expanded keyboard

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    Lotus 1-2-3 sold very well before the 101/102 key keyboard. – grahamj42 Jan 17 at 0:12
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    @grahamj42 People will live with inconveniences, but that doesn't mean they weren't clamoring for something better. – Barmar Jan 17 at 0:44
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    Holding down the Shift key to temporarily switch between navigation and data entry seems like it would be a little bit easier than toggling NumLock (and remembering the current state). – jamesdlin Jan 17 at 21:24
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    @jamesdlin is correct. With 1-2-3 e.g., a fast user would use Shift to flip modes back and forth temporarily. Entering numbers vertically didn’t even require Shifting because you just used Enter to go down. And even when I finally got a 101-Enhanced keyboard, I would still use the Num pad for bulk number entry + navigation, because it was faster! The right hand didn’t need to move anywhere. I’ve lost that speed habit over the years though. And I knew plenty of people without the hand coordination to do that, so it wasn’t universal. – Euro Micelli Jan 18 at 5:01
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I originally thought 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 (hence my confusion).

However after a bit more research (thanks due to @grahamj42) it seems that later IBM AT (aka 5170 model 339 launched April 1986) had this keyboard style (model 319 didn't) which required a BIOS update to support them.

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 grateful.

The IBM 8Mhz AT launch PR release (2nd April 1986) says this:

The cursor and screen control keys have been separated from the numeric pad, dedicating the numeric pad to numeric input. A division sign key and an additional enter key have been added to the numeric pad. As with the current IBM Personal Computer keyboards, the numeric pad may also be used for cursor and screen control when not in numlock mode.

The cursor control keys are arranged in the inverted T arrangement. Insert, delete, home, end, page up and page down keys are separated from the numeric pad and located above the dedicated cursor control keys.

So they at least thought to mention some of the advantages.

http://www-01.ibm.com/common/ssi/ShowDoc.wss?docURL=/common/ssi/rep_ca/2/897/ENUS186-052/index.html&request_locale=en

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    I was puzzled by the description IBM RT PC. In the UK, it was known as the IBM 6150. – grahamj42 Jan 17 at 0:02
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    The Wikipedia article en.wikipedia.org/wiki/Model_M_keyboard says the 101-key keyboard was copyright 1984, first sold 1985. – grahamj42 Jan 17 at 0:09
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    Wikipedia suggests it was available for the AT before the PS/2 launched, but I don't remember ever seeing one in the wild (wikipedia suggests support was added in the November 85 BIOS). – PeterI Jan 17 at 12:42

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