Animated walls are implemented using wall texture names that are built-in to the Doom rendering engine. At initialization time, the engine finds each texture name listed in the “First” column below (in the TEXTURE1 or TEXTURE2 lump), and links it with all following texture names up to and including the name in the “Last” column. At rendering time, a linked texture name in a sidedef of a wall is changed to the next linked name about three times a second, wrapping around to “First” after “Last”. (Scrolling walls are another form of animation; any wall with any texture can be made scrolling by assigning the appropriate linedef type.)