Brushes can be made using 3 kinds of solidity, not counting a substract as solidity, but more as some variance on solidity. A substracted brush is solid too just like an added brush but is sculpted out of the world, just like you would make a hole in a piece of wood, but it's shape will still be solid like a brush you would add. The purpose of this tutorial is merely to explain the types of solidity for added brushes and the use of these solidity types. The Solid Brush or ADD as it says, is completely solid, this means that it will block everything, from light to projectiles, to players. These solid brushes are used for the creation of walking surfaces and walls. For short, understand that they are used for the basic important structures of the map. All important structures like floor, ground, rock, walls, bridges and so on, are solid. These are the BLUE BRUSHES. If you have to make decorations, don't use solid as solidity type, as solid brushes cut the BSP. The engine calculates what should be solid walk or block areas, therefor the engine will cut in pieces the BSP, and this is done to cut the complete map area in several blocks to calculate them as separated modulas to enhance and fasten up this calculation. This is also done with as purpose, to compute only the immediately used area by the players, and not the other areas of the map, being unused at that moment. And here comes in the use of zones and zone-separations with the use of the Zone Actor. This is also why Semi-solid and non-solid brushes will not cut the BSP.This is also why i recommend, and i insist, don't use solid brushes for all brushes. Try to limit the use of Solid brushes to only the basic important walk and block structures. in order to break up the BSP in as less pieces as really is necesary. This will speed up the computation of your level, and also possibly avoid HOM created by these useless "solid" brushes you would have used. Semi-solid brushes are used for all decorations that need to be solid but do not need to cut the BSP and therefor make the area to be recalculated. like some stair ramp or bridge ramp, they need to block the player, but they don'tneed to cut the BSP as they are not an important walk surface. Semi-solids block light, players, but don't cut the BSP. When making a semi-solid brush touching another brush, say an add, non-solid or even another semi-solid, it may happen that this semi-solid will lose some polygon or becomes partialy invisible at these touch points with the other brushes. This also is the source of HOM. I will explain how to avoid this in next section because it is also linked to other facts. When you make a Semi-solid "Masked" decoration, it WILL also happen that the masked sections of this brush will allow you to see into nothingness. Well, this is less difficult to explain as it seems. As a masked sections purpose is to see-through, well that's what it does. Now what happened is that the masked polygon is touching the other solid polygon and that's where you can see through, as if you could see behind the solid brush but as the backside of this brush is nothing, you see that nothing. Now, to avoid this weird view, you simply have to make the semisold masked brush not touching the other brush, where the masked section touched it. Example, if the masked polygons are vertical for a masked ramp, and the brush is touching the floor, then reduce its height, in order to make it not touch the floor, that way the masked polygon can only see through itself and stops at the bottom of the ramp decoration brush itself, making this bottom end the view. If the brush was touching the other brush, then the ramp bottom was not calculated as touching the floor, but rather calculated as melted togheter with the other brush and forming a complete shape, and that's why you culd see through it. If you intend to make elements that have to look absolutely as if glued to other brushes, then you will have to reduce their size with minimal values. As for the ramp example, if you need it to be 64 high, then make it 62, these values are soo small that the player will barely be able to see this very small gap, while it allows the brush to act correctly. For possible HOM creating brushes you can try the same thing, it works the same way. You have a semi-solid decoration, but this time not masked, and it still creates HOM. This is because it touches another brush, let's say for instance, the floor. just lift it from the floor with a small value like 2 or4. Or else make this decoration smaller. Like if it was a cube crate for 128 *128, then make it 126 high, like 126 *128, you can make it 126 * 126 if you like, but only the height has importance here, making it not touvh the floor. Again don't make'm touch anything else Semi-solids are used for ICH (Invisible Collision Hull) creation too. ! ! ! try to make ICH shapes as simple as possible. these ICH brushes are invisible in game and don't need a complex shape If you use a complex shape it will reduce computation speed for nothing. You may set true the translucent flag for it, in order to be able to see them in the editor Switch to real time preview to activate their invisibility. NON-SOLID brushes as it says so well, block nothing at all. Their only purpose is to create fluid zones or fake elements that the player is not supposed to touch in any case. Like a lamp decoration at the ceiling, not one single player has to play the spiderman and sneak around at the ceiling, so he will never touch the ceilinglight. all decorations that are behind masked brushes like decoration pipes and machines will never be touched, so make'm non-solid. Small elements that are so thin or almost flat as wall decoration, will impossibly be touched by the player, or will never show that you actually go through them, make these non-solid too, this will reduce HOM possibilities and will fasten up computation, as non-solids never break up the BSP, and that's why we use non-solid for "Real decorations". By real decorations i mean all elements that are in map to enhance the look of it, details like ceilinglights, pipes and such, that you could replace with meshes if you had the chance to make and use meshes, but prefer use brush instead, make'm non-solid. If you wanted to create mountains in your fake skybox, use non-solids. If you intend to create non-reachable mountains in your map use non-solids and use semi-solid ICH to block players to reach these areas.