Customize accents when holding down a key I just noticed that if I hold down a key in mac when typing, that I get some custom bar that let's me choose between accents. 

I'm a mathematician, and I like writing unicode math symbols directly (like α). Is there a way that I can customise the toolbar that pops-up, so that I can put my own mathematical unicode symbols in this toolbar ?
Navigate to /System/Library/Input Methods/PressAndHold.app/Contents/Resources/.
Backup the Keyboard-en.plist file.
Append the following to the Keyboard-en.plist file (before the closing &lt;/dict&gt;&lt;/plist&gt;), changing the keycaps to be your desired space-delimited list of characters.

&lt;key&gt;Roman-Accent-t&lt;/key&gt;
&lt;dict&gt;
    &lt;key&gt;Direction&lt;/key&gt;
    &lt;string&gt;right&lt;/string&gt;
    &lt;key&gt;Keycaps&lt;/key&gt;
    &lt;string&gt;× ÷ √ π ∴ ∵ ≈ ≠ ∞&lt;/string&gt;
    &lt;key&gt;Strings&lt;/key&gt;
    &lt;string&gt;× ÷ √ π ∴ ∵ ≈ ≠ ∞&lt;/string&gt;
&lt;/dict&gt;



Feel free to change the Roman-Accent-t to any character you wish, providing it does not already have Press &amp; Hold keys assigned. Also see shortening the Press &amp; Hold time if you're using it regularly.