Generally, YAML files should be referred to by extension ".yml" (instead of ".yaml", if they have one).
Currently, any remaining use of the extension ".yaml" seems (in my view) slightly silly.
Commonly, the letter X is used for a great variety of meanings including "cross", "extensible", "variable", etc. (e.g. in XML). On the other hand, Y (e.g. in YML or YAML) carries no such extra baggage.
Relatively speaking, Y's use is rather uncommon. The acronym YACC comes to mind (for "Yet another compiler compiler"—BTW, its extension is simply ".y" when generating C language files). Actually, YACC assists more than it conflicts, because historically (as you may know) YAML was an acronym for the similar phrase, "Yet another markup language".
Linguistically speaking, therefore, the acronym "XML" has (in a way) only two informative letters. By contrast, all three of the letters of "YML" are fully informative. Indeed, the existing set of acronyms beginning with Y seems extremely small.
Per Wikipedia's article on information theory, "Abstractly, information can be thought of as the resolution of uncertainty." By implication, this is why a four letter YAML file extension feels greatly overspecified.
For the sake of completeness (though less important), this Ansible project comment (regarding their choice of extension) exemplifies other motivations:
> Three letter extensions owe historical relevance from DOS.
> They also save typing.
But these last two aren't the important reasons—instead, see above.
This comment (from March, 2014 in the Symphony project) also seems relevant:
> Once you have two of the biggest industry players doing ".yml" as the extension, it is the standard.
Information from the YAML.org website dates the organization's most recent activity to 2009 (approximately). Evidence of this comes from:
1. > © 2001-2006 YAML.org All Rights Reserved
2. > [The latest] News:
3. > [The latest] YAML Resources:
> YAML 1.2 (3rd Edition): http://yaml.org/spec/1.2/spec.html
Their latest specification (1.2 above) is from 2009, presumably before much of YAML's worldwide adoption:
> YAML Ain’t Markup Language (YAML™) Version 1.2
> 3rd Edition, Patched at 2009-10-01
Even their latest news item, the above-referenced JS-YAML, uses BTW the extension '.yml":
> var doc = require('/home/ixti/example.yml');
Therefore the staleness of YAML.org's information should greatly lessen the impact today from the recommendation in their FAQ:
> Is there an official extension for YAML files?
> 1. Please use ".yaml" when possible.
Here is a URL for web-searching the usage of the two YAML data file extensions (.yml and .yaml). I found time-wasting discussions of which extension to use here, here and here. Apparently these discussions are a result of the official organization exerting its influence uphill, against prevailing practice.
Maybe the public should complain. Maybe YAML.org would listen!
Here are merely some (of the many existing) usage examples of the shorter extension ".yml" in common use:
> -- NOTES:
> -- 1. The YAML file extension usually is ".yml"
> If you want a YAML file extension of .yaml (instead of .yml), you have to configure that.
> The default YAML for most projects seems to be .yml, instead of .yaml.
> In NetBeans in particular, the YAML wizard only allows creating a file with a .yml extension
> All mapping documents should get the extension ”.dcm.yml” to identify it as a Doctrine mapping file.
> The Symfony project sponsored a driver that simplifies usage of the YAML Driver. The changes between the original driver are:
> File Extension is .orm.yml
> Filenames are shortened, “MyProject\Entities\User” will become User.orm.yml
> $driver->setGlobalBasename('global'); // global.orm.yml
> As a quick start, here is a small example document that makes use of several common elements:
> # Doctrine.Tests.ORM.Mapping.User.dcm.yml
> NOTE: YAML files more often use the .YML extension.
> Configuration files should end in .yml, not .yaml
> That's the standard file ending, and should be consistent with expectations.
> Posted by cweagans on March 6, 2013
> .info files are now .info.yml files
The only counterexample I have found (using ".yaml") comes from cPanel (in their EasyApache interface, apparently a cPanel product). Generally, cPanel seems somewhat "stuffy" and old-fashioned (i.e., likely to prefer the recommendation of an official organization over common usage). Per cPanel's EasyApacheHowToMoveProfiles:
> Profile are located in the /var/cpanel/easy/apache/profile/custom directory. The filename will be identical to the name you save it with, plus the .yaml file extension.
EDIT: The above reference is from the Wayback machine. However, as of August, 2016, we can still see cPanel using only extension ".yaml".
Copyright (c) 2013 Mark D. Blackwell.