11. key modifiers & key level choosers

key modifiers

a key modifier is a special key on the keyboard that changes the way
the pressing of another key will be interpreted.

key
- shift + key
- ctrl + key
- alt + key
- AltGr + key [alt graphics]
- super (winkey/metakey/ etc) + key
- on laptops : often also Fn + key

These modifiers can be combined with each other ( ctrl+ alt + key , ctrl+ shift + alt + key , etc ) to allow for many more combinations.

Key modifiers can be used for 2 different purposes :
1. modify the key sequence produced ( shift + a ==> A )
this first type is called a (key) level chooser (only in linux ?)
2. launch actions

key levels choosers defined

These use some of the key modifiers ,
not ctrl and not alt keys which are strictly action related
Standard : shift , altgr , shift + altgr ( = levels 2 > 4 )
For level 5 you can specify an extra modifier yourself
| level | Modifier | keys produced |
| ----- | ------------- | ------------------------------------------------------------- |
| 1 | - | lowercase letters, top row numbers (qwerty) + symbols (azerty) (1) |
| 2 | shift | uppercase letters, top row symbols (qwerty) + numbers (azerty) (1) |
| 3 | AltGr | extra symbols not printed on the keyboard, some dead keys |
| 4 | shift + AltGr | more symbols not printed , more dead keys |
| 5 | {custom} | |

(1) as printed on keyboard

Thus , “third level of A” is another way of saying : press AltGr + A
( adding a third modifier adds four more levels ? )

excellent posts :
https://itectec.com/ubuntu/ubuntu-what-does-key-to-choose-5th-level-in-gnome-keyboard-properties-do/
https://ubuntu-mate.community/t/make-your-own-custom-keyboard-layout-for-linux/19733