Another technological Cheshire cat?

June 08, 2017

Max The Magnificent-June 08, 2017

Have you ever wondered why the standard rack size for electronic gear is 19" wide, while the vertical unit is 1.75" (so a 1U rack is 1.75" tall, a 2U rack is 3.5" tall, and so on)?

No? I must admit that I'm in the same boat. This is one of those things I simply accepted without thinking about, but this choice of sizes must have started somewhere.

Here's another question: when do you think these sizes were first defined? If you'd asked me this yesterday, my thought process would probably have been something along the following lines:

  1. I always think of racks in terms of servers (and switches and stuff).
  2. The first servers of which I'm aware started to be deployed sometime in the 1970s to handle things like bulletin boards.
  3. Thus, the 19" and 1.75" values must date from around the 1970s, QED!

I wondered if this was the way others would think, so I bounced over to the next bay to ask my chum Ivan, and he replied that he was aware of 19" x 1.75" racks being used in military and radio equipment from the 1950s and 1960s, but -- like me -- he had no idea as to where these values originated.

The reason I'm waffling on about this here is that I received a message from Embedded.com Community member Perl_Geek saying:

If you're looking for a trivia question that might become a topic, why is the standard rack size for electronic gear 19" (and the vertical unit 1.75")? I have seen an advertisement dating back to the 1930s reflecting these dimensions. Is this another "technological Cheshire cat"?

One of the Cheshire cat's distinguishing features -- apart from the fact it can talk, of course -- is that from time to time its body disappears, with the last thing visible being its iconic grin. You may not be surprised to hear that this set me off wondering about the origin of the phrase "Cheshire cat," which most of us associate with Alice's Adventures in Wonderland by Lewis Carroll. According to this Wikipedia entry, however, the Cheshire Cat actually predates Carroll's 1865 novel, but we digress...

I asked Perl_Geek if he could remember where he saw the advertisement in question, and he pointed me at this Hammond Radio Transmitting Equipment catalog from 1935. On page 18 in the catalog (page 21 in the PDF) we see we see adverts for Racks and Panels that include the following:

1471 --  1 3/4 in. 1 rack unit    $1.10
1472 --  3 1/2 in. 2 rack units   $1.55
1473 --  5 1/4 in. 3 rack units   $2.10
1474 --  7     in. 4 rack units   $2.75
1475 --  8 3/4 in. 5 rack units   $3.10
1476 -- 10 1/2 in. 6 rack units   $3.80
1477 -- 12 1/4 in. 7 rack units   $4.45

Hmmm. Interesting. So 19" racks were firmly established in 1935; the mystery grows...


(Source: Hammond Radio Transmitting Equipment Catalog G-48)

Now, we could all have a quick Google (it's alright so long as no one is looking), but rather than that I thought I'd throw this question out to the experts -- yes, I mean you: sit up straight, comb your hair, and try to look intelligent (or, at least, sentient) -- and allow you to dazzle the rest of us with your mastery of the mystic arts. So, do you know why, when, and where the 19" and 1.75" units came into being?

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