system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do so many password requirements specifically forbid spaces?
gpt
Because they have always had that requirement. 

Many years ago, passwords were stored as plain text in a database, or sometimes in a text file. When the computer read the password back from storage to compare it against the one it just got from a logging in user, a space in the password could be interpreted as the end of the password. This caused issues when comparing the passed password to the stored one. 

Issues could also be caused by certain special characters ( like ‘ and $) being used, that also meant something to the computer program, causing confusion and errors. So many times some special characters were not allowed either. Also, many programs had a tight limit on their storage space, so password length was also restricted. 

Since then, standards have changed, passwords normally aren’t stored directly anymore (T-Mobile????) Instead, a password is fed into a complex math problem and the result is stored. This means that most of the limitations that were imposed by the previous issues no longer apply. Unfortunately, the past rules are still used, even though they’re not needed, because they worked before and people are used to them.