While Subversion allows you to move around versioned files and directories without any loss of information, and even provides ways of moving whole sets of versioned history from one repository to another, doing so can greatly disrupt the workflow of those who access the repository often and come to expect things to be at certain locations. So before creating a new repository, try to peer into the future a bit; plan ahead before placing your data under version control. By conscientiously “laying out” your repository or repositories and their versioned contents ahead of time, you can prevent many future headaches.
Let's assume that as repository administrator, you will be responsible for supporting the version control system for several projects. Your first decision is whether to use a single repository for multiple projects, or to give each project its own repository, or some compromise of these two.
There are benefits to using a single repository for multiple projects, most obviously the lack of duplicated maintenance. A single repository means that there is one set of hook programs, one thing to routinely backup, one thing to dump and load if Subversion releases an incompatible new version, and so on. Also, you can move data between projects easily, and without losing any historical versioning information.
The downside of using a single repository is that different projects may have different requirements in terms of the repository event triggers, such as needing to send commit notification emails to different mailing lists, or having different definitions about what does and does not constitute a legitimate commit. These aren't insurmountable problems, of course—it just means that all of your hook scripts have to be sensitive to the layout of your repository rather than assuming that the whole repository is associated with a single group of people. Also, remember that Subversion uses repository-global revision numbers. While those numbers don't have any particular magical powers, some folks still don't like the fact that even though no changes have been made to their project lately, the youngest revision number for the repository keeps climbing because other projects are actively adding new revisions. 
A middle-ground approach can be taken, too. For example, projects can be grouped by how well they relate to each other. You might have a few repositories with a handful of projects in each repository. That way, projects that are likely to want to share data can do so easily, and as new revisions are added to the repository, at least the developers know that those new revisions are at least remotely related to everyone who uses that repository.
After deciding how to organize your projects with respect
to repositories, you'll probably want to think about directory
hierarchies within the repositories themselves. Because
Subversion uses regular directory copies for branching and
tagging (see Chapter 4, Branching and Merging), the
Subversion community recommends that you choose a repository
location for each project
root—the “top-most” directory
which contains data related to that project—and then
create three subdirectories beneath that root:
trunk, meaning the directory under which
the main project development occurs;
branches, which is a directory in which
to create various named branches of the main development line;
tags, which is a collection of tree
snapshots that are created, and perhaps destroyed, but never
For example, your repository might look like:
/ calc/ trunk/ tags/ branches/ calendar/ trunk/ tags/ branches/ spreadsheet/ trunk/ tags/ branches/ …
Note that it doesn't matter where in your repository each project root is. If you have only one project per repository, the logical place to put each project root is at the root of that project's respective repository. If you have multiple projects, you might want to arrange them in groups inside the repository, perhaps putting projects with similar goals or shared code in the same subdirectory, or maybe just grouping them alphabetically. Such an arrangement might look like:
/ utils/ calc/ trunk/ tags/ branches/ calendar/ trunk/ tags/ branches/ … office/ spreadsheet/ trunk/ tags/ branches/ …
Lay out your repository in whatever way you see fit. Subversion does not expect or enforce a particular layout—in its eyes, a directory is a directory is a directory. Ultimately, you should choose the repository arrangement that meets the needs of the people who work on the projects that live there.
In the name of full disclosure, though, we'll mention
another very common layout. In this layout, the
branches directories live in the root
directory of your repository, and your projects are in
subdirectories beneath those, like:
/ trunk/ calc/ calendar/ spreadsheet/ … tags/ calc/ calendar/ spreadsheet/ … branches/ calc/ calendar/ spreadsheet/ …
There's nothing particularly incorrect about such a layout, but it may or may not seem as intuitive for your users. Especially in large, multi-project situations with many users, those users may tend to be familiar with only one or two of the projects in the repository. But the projects-as-branch-siblings tends to de-emphasize project individuality and focus on the entire set of projects as a single entity. That's a social issue though. We like our originally suggested arrangement for purely practical reasons—it's easier to ask about (or modify, or migrate elsewhere) the entire history of a single project when there's a single repository path that holds the entire history—past, present, tagged, and branched—for that project and that project alone.
 Whether founded in ignorance or in poorly considered concepts about how to derive legitimate software development metrics, global revision numbers are a silly thing to fear, and not the kind of thing you should weigh when deciding how to arrange your projects and repositories.
branches trio are sometimes referred
to as “the TTB directories”.