Maybe they are hankering for some good old bit screeching. 

Must be a default, I never set it. 

What causes z-fighting and how does one avoid it in creating a map? 

Z-fighting is cause when faces of brushes overlap. In general this will only happen with things like func_door, func_plat etc.

So if you make a door or a platform make sure that none of the visible faces are level with other visible faces. This means that you should model in recesses for your platforms and doors to go into in most cases. 

So what about having two world brushes with overlapping faces that the player can see? Will this ever result in z-fighting? And if not, is there any rule to which of the two faces (i.e. which texture) is seen in-game? 

with world brushes you should not have them overlapping in such a way that parts of faces which will be seen are flush. You don't know and then engine does not know, hence the z-fighting. I think the gl_zfix thing only sets worldspawn to a higher prio than entities. 

Ok, thanks for clarifying. I'm working on structure where it's kind of tricky not to have any visible surfaces overlap, especially without splitting the brushes into tiny components -- but I guess I'll have to figure it out somehow, then.

Thanks again! 

Feel free to upload a work in progress of your .map and I can have a look at it and let you know what might be problematic. 

That's very kind of you, but at the moment I'm still too embarrassed to show anything I've made.

I might take you up on the offer when things look less messy ... but that's going to take a while at my pace. 

shouldn't happen. Here's a test:
http://imgur.com/a/McOtQ#Ce5LmYE

qbsp has a step where it CSG-subtracts each brush against each other brush. I think they're done in map file order, so the first brush I added there clips away the corner of the second brush.

The gl_zfix cvar in quakespasm was pushing each face of bmodels (so func_wall/func_door, etc.) out by a small amount, but it's disabled now becuase it caused arifacts making some secrets visible, and caused mappers to put z-fighting in their maps inadvertently. 

No problem :) 

That is still kinda bad practise though, right? Plopping worldspawn brushes together like that. Esp. since you do not want to keep track of which brushes were placed earlier.

So in most cases like that you should still cut one of the brushes up into smaller parts, right? 

yeah, it's best to avoid - messy map file. But if it's hard to avoid overlapping world brushes, the compiler will deal with it. 

Thanks for the extensive responses, ericw and ptoing. 

That's what this place is here for :D 

I've noticed that on some angled brush-faces, the brick texture I'm using is warped -- as in, the vertical lines on the texture (the short part of the bricks) are diagonal instead of vertical. No amount of changing the texture angle/size/X/Y-offset seems to help...

Is there any way of fixing/avoiding this? I'm using TrenchBroom 2 on Linux. 

I dunno about TB2, but Jackhammer has very good texture alignment tools and there is a Linux build, so maybe worth looking into that. 

I *think* that's something the "Valve 220" texture alignment fixes. And I *think* TrenchBroom supports it.

At the moment I use Netradiant, but I seem to remember somebody here sending me a map a year or so ago when I was trying TB1 that showed how it looked, and I remember being surprised how much better it was at aligning textures on angled and rotated surfaces.

Make a backup copy of your map if you try something like that, it may not be reversible. 

I can't believe I've been mapping all this time and just discovered this. Makes positioning info_intermission easy.

Turn on noclip and find a good view. Open the console and enter the "viewpos" command (no quotes). Write down the numbers:

(origin) mangle

Enter these in your map for an info_intermission - Done. 

That's a result of the way textures are projected onto faces in Quake. Your only option is to use the Valve 220 map format, which TB2 supports. However there's no conversion function yet, but I could implement one. 

Thanks very much, ptoing, Rick and SleepwalkR, for your responses.

SleepwalkR, how does one go about using the Valve 220 format? I can't see any option for enabling it. Is it enabled by default? Or is it something one specifies when compiling? 

when you create a new map. In the game selection dialog, when you select Quake, you see a dropdown list below the game list where you can choose between Standard and Valve 220 map formats. 

Does that mean I'm now stuck with standard format in the current map I'm working on, at least until you implement conversion? D'oh.

I tried opening a new .map in Valve format and copy&paste-ing my current map into the new file, but it doesn't work -- it says
Unable to parse clipboard contents: (line 2, column 1)Expected '(' or ')', but got 'classname'
I'm guessing that's because you can't (currently) copy&paste between the two formats in TB2? 

First, I need to write a converter. Maybe I can squeeze one in tonight, because converting from Standard to Valve 220 is easy. The converse isn't, though.