system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why was a seemingly random set of notes chosen as the "standard" tuning for a guitar? When and how was this tuning decided upon?

I know a little bit about musical theory and have been playing guitar for most of my life, albeit casually, yet this question only occurred to me recently.
gpt
The guitar is a latecomer in a long line of string instruments that are tuned "4-3-4" (after the intervals separating the strings).  There's a lot of ancient 4-string instruments tuned so that the interval between the two middle strings is a major third, and the interval between an outer string and its adjacent string is a major fourth.  The ukulele is tuned like this, for example.  And the guitar is basically like a larger, six-string ukulele with two extra bass strings tuned in fourths.

But still, why?  Well, first, let's consider the alternatives:

* **Open tuning:** the guitar is tuned so that when all the strings are played open together, some specific chord is played out.  This is a traditional alternative tuning as well, used often in blues.  Some instruments like pedal steel guitar are also normally tuned like this.
* **Fourths tuning:** all strings are tuned in fourths.  Not nearly as common, but a few musicians use it.

Open tuning makes it really easy to play chords for a chosen key using all strings (which gives a fuller sound), but playing scales and transposing become more difficult.  Fourths tuning makes it easy to play scales and transpose, but chords are much more limited (you're basically stuck with four strings at a time).

The standard tuning offers a balance between those two alternatives.  There's a bunch of easy open chord shapes for more than one key.  You also get easy five-or-six string barre chords all over the neck for any key.  Scales are somewhat harder than fourths tuning, but not nearly as hard as in open tuning.