This type of door raises its upper half into the ceiling and lowers its bottom half into the floor when opened. It's been dubbed "Clamshell Door". It is just as simple to construct as the Ceiling and Floor Doors, but it looks cooler.
Construct the door sector and tag it [0,22].
Enter 3D mode and texture the door's frame. You may need to flip the textures around with F to make them look proper. Place an M[StopSound,StartSound] ( in the door sector. I recommend Speed[0,Speed], S[Delay,10])M[0,166]. Raise the floor and lower the ceiling so they meet in the middle of the door.
When you texture the door, you may notice that it is flipped incorrectly. To fix the texture so it looks proper, point the mouse cursor at the top half of the door and press 2. Now you can edit the two halves separately. You can paste the texture to both halves of the door and use the F key to flip them around. Once you've done that, your door is complete!
Note: The door frame will move with the door no matter which way you orientate it. This is because a split door moves both the ceiling and the floor. To fix this problem, make two extremely thin sectors attached to each side of the door, and make sure they are not tagged. Texture them with door frame textures. Now the door frames will be separate from the door itself, so they won't move with the door anymore. Check the Split Door in TUTORIAL.MAP for an example.