Stacking xorg.conf.d snippets

We've had xorg.conf.d snippets for quite a while now (released with X Server 1.8 in April 2010). Many people use them as a single configuration that's spread across multiple files, but they also can be merged and rely on each other. The order they are applied is the lexical sort order of the directory, so I recommend always prefixing them with a number. But even within a single snippet, you can rely on the stacking. Let me give you an example:

$ cat /usr/share/X11/xorg.conf.d/10-evdev.conf
Section "InputClass"
    Identifier "evdev touchpad catchall"
    MatchIsTouchpad "on"
    MatchDevicePath "/dev/input/event*"
    Driver "evdev"
EndSection

$ cat /usr/share/X11/xorg.conf.d/50-synaptics.conf
Section "InputClass"
    Identifier "touchpad catchall"
    MatchIsTouchpad "on"
    MatchDevicePath "/dev/input/event*"
    Driver "synaptics"
EndSection

The first one applies the evdev driver to anything that looks like a touchpad. The second one, sorted later, overwrites this setting with the synaptics driver. Now, the second file also has a couple of other options:

Section "InputClass"
    Identifier "Default clickpad buttons"
    MatchDriver "synaptics"
    Option "SoftButtonAreas" "50% 0 82% 0 0 0 0 0"
EndSection

This option says "if the device is assigned the synaptics driver, merge the SoftButtonAreas option". And we have another one, from ages ago (which isn't actually needed anymore):

Section "InputClass"
    Identifier "Disable clickpad buttons on Apple touchpads"
    MatchProduct "Apple Wireless Trackpad"
    MatchDriver "synaptics"
    Option "ClickPad" "on"
EndSection

This adds on top of the other two, provided your device has the name and is assigned the synaptics driver.

The takeaway of this is that when you have your own xorg.conf.d snippet, there is almost never a need for you to write more than a 5-line snippet merging exactly that one or two options you want. Let the system take care of the rest.