+7  A: 

I would use .inc (meaning include file) or .txt.

Daniel A. White
+1 on .txt... because without the full valid structure <head><body><etc>.. it's just a text file.
datacop
<head>, <body> are HTML tags used to define sections of a document written in HTML. Without them the HTML doesn't describe a web page, but is still HTML.
Peter Perháč
But its not XHTML.
Daniel A. White
the comment above was @datacop.
Peter Perháč
@Daniel, I'm using HTML 4.01, not XHTML.
system PAUSE
@system - I was referring in general case.
Daniel A. White
http://msdn.microsoft.com/en-us/library/ms972100.aspx - Looks like there's a solid precedent for this, server-side at least. I'm not using ASP, but the difference between .html and .inc should be obvious to other coders who come from the ASP world.
system PAUSE
+1  A: 

I generally use .inc (include file) or .shtml (shared HTML). I've never heard of .pht, though.

Matt Grande
.shtml is Server HTML
Charlie Somerville
+3  A: 

As the files are really written in Hyper Text Mark-up Language, I think it's quite valid to give them the .html extension. Consider just calling the Directory something like "divs" or "panels", "forms", etc...

Peter Perháč
Minor annoyance: If I do this, Eclipse shows a validation warning.
Nicolas Raoul
A: 

I've only seen the ".phtml" extension before but I've never used it. I always use ".html", like any other ".html" file. Like MasterPeter said the file is written in HTML, so I think it's correct.

HED
A: 

Another extension, ".fhtml", appears to be used by some outdated Macromedia product, and at least one other software package uses it to mean "fragment HTML".

Having a precedent is nice, but unfortunately the .fhtml suffix also used to indicate HTML templates with embedded Factor code. That kind of ambiguity is annoying.

system PAUSE
+1  A: 

I use "inc". I don't see any reason to worry about a thing like that. Call it whatever you want, as long as you get the benefit of looking at the files in a directory listing and not being confused.

One example where I don't use "inc" would be when there are includes inside includes, say your outer page includes a nav include, which then includes a third file. In that case you have to stay with whatever your server expects the extension to be for server-parsed files.

AmbroseChapel
+1  A: 

I generally use .inc or .tpl (how come nobody mentioned this?).