When I moved from Windows to OS X the first thing that I noticed was that the titlebar buttons are on the left instead of right side. It took me some time to get used to it.
Is there a reason why they put them on the left side?
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In the original MacOS versions (pre-10), there was a close button in the top left and maximize (if available) in the top right, as shown here:
Putting them together seems like a reasonable choice to tidy up the interface. So, perhaps the real question is why was the close button in the top left corner originally?
My best guess: Mac users use Command-Q (or Apple-Q) to close a program and since the Q key is on the top-left of the keyboard, putting the close button on the top-left of window reinforced its purpose and made for a slightly more consistent experience overall.
Steve Jobs was fairly inspired by Xerox's Xerox Star, which was the first to introduce a User Interface in their system.
Demo video: https://www.youtube.com/watch?v=QYvxgNhUwBk
Xerox's Xerox Star was poorly marketed, and hence wasn't well known.
With the inspiration of the Xerox Star, Apple launched Mac OS 1.0 and placed the Window Commands/Actions to the Left, since Xerox too had most of their primary controls on the left.
However, there hasn't been a pure reason as to why where they placed on the left since no one really cared about UX back then, and it was placed in that place for their convenience.
Is there a reason they're on the left?
Yes. It's that the ultimate button in this little micro "dialog" box (Window title bar) is the destructive action of closing the window, and Apple determined the ideal order of actions to be from left to right:
| Destructive || Neutral || Constructive |
Since the button on the far left closes the window, and is therefore the most destructive possible button in a window based GUI OS, it should be on the far left of the Window status bar that is its container when using these rules of lateral action distribution. This fundamental design choice informed the decision to put the close window button on the far left, the other two actions (minimise and maximise) are placed beside it because that's now the area that is the "dialog box action area" of the Window Title Bar.
For a brief while we had the ideal of a super-maximise-to-a-new-screen-space on the top right, double tapping the middle section of the Window Title Bar to minimise and far left being the destructive close button with the minimise and maximise buttons next to it.
But then a (wrong headed) desire to put the download button and iOS sharing button in the top right of Safari seemed to override this ideal across all of Mac OS X, and we lost the normal and rightful maximise button in this little trio. It's been replaced with the fullscreen, new space button. I'm pretty sure this little mess wouldn't have happened under Steve Jobs' watch.
In answer to your original question, yes everything about OS GUI design is intentional, and has reasoning behind both the intentions and the decided design. It's just not always transparent, and sometimes not good intentions, like the desire to transplant the ideas of iOS sharing button placement to Mac OS X.
What's the reason the buttons are on the left of the window?
it's because most people are right handed and clicking on the left side feels "wrong'. It's counter-intuitive for something as important as closing an interaction window to be on the "wrong" side. So a user is less likely to close a window by accident.
Where I'm coming from: I used to accidentally close windows on Windows all the time when I was trying to maximize the window because I would move too fast. Clicking "close' on the left side on Mac has forced me to slow down and only close windows if I really want to :)