If you like spamming, a recent feature update of Apple may annoy you. The default behavior for long key-presses was a repetition of the respective character. If you now press a consonant like p nothing happens, if you press a vowel for a little longer, you may see a menu to select accentuated alternative characters, like
If you want to turn off the accent selector menu and re-enable key repetition, you can restore this setting. Unfortunately there is no option in the system preferences for this really useful option. But setting the defaults via the Terminal app just does the job. Open a Terminal window and copy and paste the following line:
defaults write -g ApplePressAndHoldEnabled -bool false
After you hit enter, the change should take place. However, most programs like Skype, Chrome and Text Edit must be restarted so that the change becomes effective. If you desire a system-wide change, restart your computer.
If you don't like the effect, you can undo the change by executing the following command:
defaults write -g ApplePressAndHoldEnabled -bool true
This trick should work on any OS-X version, including Mavericks, Yosemite, El Capitan and Sierra.