The Stacks project I have a question concerning the admirable Stacks Project.

Which comparable projects are there:

*

*approach-wise: "an open source textbook on algebraic stacks and the algebraic geometry that is needed to define them", i.e. projects
as "an open source textbook on X and the Y that is needed to define
(and understand) X"


*technical-wise [Side question: which technical framework(s) is the Stacks Project based on?]


*tag-system-wise (using the same tag system as the Stacks Project)


*tag-wise (with a significant number of tags shared with the Stacks Project)?

The question concerns also the possible interoperability of several such projects.
 A: tag-system-wise Anyone can use Gerby, with a similar tag system. The actual tag assignment isn't dealt with by Gerby, you can set up your own conventions if you like. There might be some minor things you'll have to change if you decide to do so.
So far I'm not aware of anyone actually doing this, except for the work-in-progress Kerodon, which will be Gerby applied to the works of Jacob Lurie. There's no fixed date to go live yet, maybe in September we'll have a more definite idea about this.
Observe that the text will not be open-source (there's no need for this in the Gerby system).
tag-wise No projects share tags with the Stacks project (as there are no other projects using a similar tag system), although it is often cited. It's also not quite desirable to "share" tags, as nothing ensures uniqueness of tags then.
technical-wise This has been answered before, but feel free to ask follow-up questions (which probably would be better done via e-mail, or chat), as I'm the person responsible for the majority of the implementation (with help from @RaymondCheng).
A: approach-wise:
Open source text books on a variety of topic in mathematics (and other sciences), mainly CC BY licensed, so the content can be freely shared and adapted, even for commercial use:


*

*OpenStax

*College Open (some with a non-commercial-use restriction)

*Wikibooks
Answer to side question: Stacks is based on a LaTeX processing tool built using plastex and
a website built using Flask (see Gerby).
