Install How to use a build
For those are new here, or need a reminder.
 Where to look
The most interesting files are the dictionary files; 'apertium-xxx.xxx.dix' and 'apertium-yyy.yyy.dix' in the respective monodix folders, and 'apertium-xxx-yyy.xxx-yyy.dix' in the bidex folder.
For a fuller list of files and what they do, see the Workflow reference.
 How to access a pair
The configuration of a pair is centred in the bilingual dictionary (see above). This applies to other actions on, or accesses to, the build. For example, if you have a tool, and it is able to find a pair by browsing the system, point it at the bilingual folder. There, the tool will find everything it needs.
 How to repeatedly compile pairs
When the pair directories were set up, the bilingual pair folder was configured from the two monolingual dictionary directories.
Compiling all three folders to make corrections would be tedious. The Apertium build avoids this. The build is gathered in the bilingual folder. Go there ('apertium-xxx-yyy/'), and type this special command,
and you will see all three folders compile in the terminal output.
If you have a fast modern computer,
make -j3 langs
will attempt to compile using three cores.
Folders can be compiled individually, but you will rarely need to do this.
 For beginners: how do I use it?
I would say, ignore mention of tools like
lt-proc. Ignore the instructions you find on the Wiki, some of them date back a long way, or deal with the tools in detail.
 ...and for pros: Bash completion
You can get tab-completion on mode names (and the '--with-lang' arguments to autogen.sh) by adding Bash completion.