If you install Windows 10 using a Microsoft account, instead of a local account, it will truncate the username to five letters, and use that as the user directory and a few other things. (you can fix the user directory with this: https://superuser.com/a/955026/310715)

My question, which I can't seem to find an answer to, is why! Why is Microsoft truncating the user name to five characters for linked accounts? To put it another way: why is the truncation taking place and why the (seemingly) arbitrary length?

EDIT: this question was closed as generating primarily opinion based answers. I guess the presumption is that only Microsoft can know the reason why they chose to truncate, and why they chose five characters. But, if they've published their thinking somewhere, or someone has credentialed knowledge, I think there is still an answer out there lurking. Please, if you have information or leads, add comments.

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    @DaaBoss, but, rather than truncating at 5 chars, it would make sense to truncate at the commercial at symbol, since that separates the handle from the domain anyway. – undrline - Reinstate Monica Nov 23 '16 at 15:02
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    As you implied when you mentioned the previous question that references a similar scenario, the most graceful way to initially setup your Windows 10 profile is to first utilize a local login account, then "convert it" to your Microsoft account. In addition to creating a standard profile folder hierarchy, it also makes a difference when you check online to see which computer names are attached to your Microsoft account (for digital entitlement licenses, etc.). If you use your Microsoft account for initial setup before you rename the computer, that generic computer name is retained online. – Run5k Nov 23 '16 at 15:58
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    I wonder what happens if two usernames shared the same first 5 characters? – Stevoisiak Jun 5 '17 at 15:08
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    @AlexZhukovskiy I've just installed W10 3 times last weekend and now somehow it truncates my username to 5 letters – phuclv Aug 24 '18 at 14:31
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    "5 letters ought to be enough for anybody's name." -Gates – Amit Naidu Nov 4 '18 at 15:08

More testing is obviously required before this directly answers the question. Without more testing, this answer is incomplete at best.

(YMMV) - Advice for initially setting usernames when installing Windows 10
Anytime you create a new user on a PC, I think you should use a local account first, which sets the user's folder structure name, as in the C:\Users\[localAccount] folder. Then, if you really want to log in with, and attach a Microsoft registered email address to your PC login, it won't change your folder structure.

Since some email addresses would be extremely long, using that for the username which becomes the root folder for the currently logged in user would cause difficulty for programs and users. Often, logs are produced that would then contain that user's email address. These logs often are sent or posted by the user, which would publish that user's email address.

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    That's not the problem. I have a real Windows username for the live account like someuser which is completely irrelevant to the live email address, but after I sign in into other computers it'll be truncated to someu – phuclv Nov 23 '16 at 16:29
  • I also want to add that potentially the @ symbol could have multiple meanings aside from it's apparent character, leading to issues with folder names. As far as I remember, folder names cannot contain that particular special character. – Kaizerwolf Nov 23 '16 at 16:31
  • @Kaizerwolf @ is not a forbidden character in filenames – phuclv Apr 19 '17 at 15:39
  • lol, it's funny when I come back to something and want to upvote it again. Like @LưuVĩnhPhúc pointed out, this doesn't answer the problem I posed: why is the truncation taking place and why the arbitrary length? People have argued that the email construct has nothing to do with it. Even if it was a length issue, why davka this length? – undrline - Reinstate Monica Jun 2 '17 at 16:57
  • Clearly, there's more to this than it appears.... I'd like to improve my answer, but I was unaware of many of the issues here, let alone why Microsoft did what they did. My answer was also trying to add something useful to the advice users should use when installing Windows. My testing so far is too anecdotal and limited. However, I will start creating accounts under different scenarios to better understand. Adding to your question, asking for more outside resources to shed some light on the subject might also help. – DaaBoss Jun 5 '17 at 13:22

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