KDE Plasma

Enable compose key in system settings

This is useful when typing special characters like currency and typographic symbols like en dash, em dash etc.,

  1. Open System SettingsInput DevicesKeyboard
  2. Click the Advanced tab (see screenshot below)
  3. Select a key under Position of Compose key

Use Right CTL key as compose key

Useful shortcuts

euro
compose key + E + =

° degree
compose key + o + o

ellipsis
compose key + . + .

en dash
compose key + - + - + .

em dash
compose key + - + - + -

× multiplication symbol
compose key + x + x

More shortcuts are listed on this Wikipedia page