How to find out and interpret the LaTeX version number?

Question

When I do latex -v to find out which version of latex (or pdftex, xelatex etc., outputs look similar) I am using, I get

pdfTeX 3.14159265-2.6-1.40.16 (TeX Live 2015/Debian)
[...]

What does this version number tell me?

Answer 1

There are three version numbers here, none of which relate to LaTeX! They are:

• 3.14159265 The version of TeX on which this pdfTeX release is based
• 2.6 The version of e-TeX used
• 1.40.16 The pdfTeX release itself

Notice that pdfTeX is derived from e-TeX which is derived from Knuth's TeX (TeX90), hence the chain of version numbers for the engine. For other engines derived directly from TeX90 (XeTeX, pTeX, e-upTeX, etc.) you'll see similar version data. Notice that for LuaTeX you won't as the code is no longer 'Take TeX90 and patch' (or similar), but rather has deviated. It therefore has only it's own version data:

This is LuaTeX, Version 1.0.4 (TeX Live 2017/W32TeX)

To see the LaTeX version you need something like pdflatex \stop (escaping for your shell as necessary). I get

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017/W32TeX) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
LaTeX2e <2017-04-15>
Babel <3.9r> and hyphenation patterns for 83 language(s) loaded.
No pages of output.
Transcript written on texput.log.

for this, i.e. LaTeX2e release 2017-04-15 (a version string, not actually a date though roughly related!).

Answer 2

• Metafont version number tends to e
• TeX version number tends to pi

Other tools in the ecosystem have more classical version number

Best regards