# Frequently Asked Keymap Questions
This page covers questions people often have about keymaps. If you haven't you should read [Keymap Overview](keymap.html) first.
## What Keycodes Can I Use?
See [Basic Keycodes](keycodes.html) and [Quantum Keycodes](quantum_keycodes.html) for most of the keys you can define.
Keycodes are actually defined in [common/keycode.h](https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/keycode.h).
## `KC_SYSREQ` isn't working
Use keycode for Print Screen(`KC_PSCREEN` or `KC_PSCR`) instead of `KC_SYSREQ`. Key combination of 'Alt + Print Screen' is recognized as 'System request'.
See [issue #168](https://github.com/tmk/tmk_keyboard/issues/168) and
- http://en.wikipedia.org/wiki/Magic_SysRq_key
- http://en.wikipedia.org/wiki/System_request
## Power key doesn't work
Use `KC_PWR` instead of `KC_POWER` or vice versa.
- `KC_PWR` works with Windows and Linux, not with OSX.
- `KC_POWER` works with OSX and Linux, not with Windows.
More info: http://geekhack.org/index.php?topic=14290.msg1327264#msg1327264
## Oneshot modifier
Solves my personal 'the' problem. I often got 'the' or 'THe' wrongly instead of 'The'. Oneshot Shift mitgates this for me.
https://github.com/tmk/tmk_keyboard/issues/67
## Modifier/Layer stuck
Modifier keys or layers can be stuck unless layer switching is configured properly.
For Modifier keys and layer actions you have to place `KC_TRANS` on same position of destination layer to unregister the modifier key or return to previous layer on release event.
- https://github.com/tmk/tmk_core/blob/master/doc/keymap.md#31-momentary-switching
- http://geekhack.org/index.php?topic=57008.msg1492604#msg1492604
- https://github.com/tmk/tmk_keyboard/issues/248
## Mechanical Lock Switch Support
This feature is for *mechanical lock switch* like [this Alps one](http://deskthority.net/wiki/Alps_SKCL_Lock). You can enable it by adding this to your `config.h`:
```
#define LOCKING_SUPPORT_ENABLE
#define LOCKING_RESYNC_ENABLE
```
After enabling this feature use keycodes `KC_LCAP`, `KC_LNUM` and `KC_LSCR` in your keymap instead.
Old vintage mechanical keyboards occasionally have lock switches but modern ones don't have. ***You don't need this feature in most case and just use keycodes `KC_CAPS`, `KC_NLCK` and `KC_SLCK`.***
## Input special charactors other than ASCII like Cédille 'Ç'
NO UNIVERSAL METHOD TO INPUT THOSE WORKS OVER ALL SYSTEMS. You have to define **MACRO** in way specific to your OS or layout.
See this post for example **MACRO** code.
http://deskthority.net/workshop-f7/tmk-keyboard-firmware-collection-t4478-120.html#p195620
On **Windows** you can use `AltGr` key or **Alt code**.
- http://en.wikipedia.org/wiki/AltGr_key
- http://en.wikipedia.org/wiki/Alt_code
On **Mac** OS defines `Option` key combinations.
- http://en.wikipedia.org/wiki/Option_key#Alternative_keyboard_input
On **Xorg** you can use `compose` key, instead.
- http://en.wikipedia.org/wiki/Compose_key
And see this for **Unicode** input.
- http://en.wikipedia.org/wiki/Unicode_input
## Apple/Mac keyboard Fn
Not supported.
Apple/Mac keyboard sends keycode for Fn unlike most of other keyboards.
I think you can send Apple Fn key using Apple venter specific Page 0xff01 and usage 0x0003. But you have to change HID Report Descriptor for this, of course.
https://opensource.apple.com/source/IOHIDFamily/IOHIDFamily-606.1.7/IOHIDFamily/AppleHIDUsageTables.h
## Media control keys in Mac OSX
#### KC_MNXT and KC_MPRV does not work on Mac
Use `KC_MFFD`(`KC_MEDIA_FAST_FORWARD`) and `KC_MRWD`(`KC_MEDIA_REWIND`) instead of `KC_MNXT` and `KC_MPRV`.
See https://github.com/tmk/tmk_keyboard/issues/195
## Keys supported in Mac OSX?
Y