# Why does PDFLaTeX and XeLaTeX generates 11.955 font size for 12pt documentclass option?

I am currently working on post-processing PDFs that are generated from either pdflatex or xelatex with the 12pt documentclass option. Since I am looking at the uncompressed PDF sources, I noticed that the PDF command Tf to select the font specifies a font size of 11.955 instead of 12. I am curious: does anyone here know why?

Minimal example for XeLaTex:

% !TEX TS-program = xelatex
\documentclass[12pt]{article}

\begin{document}
Why is this font size 11.955 in PDF?
\end{document}


but typeset with xelatex --output-driver="xdvipdfmx -V4 -z0" <FILE> to obtain uncompressed PDF output. Then, looking at the .pdf file, the first object is the stream:

stream
q 1 0 0 1 72 720 cm BT /F1 11.955 Tf 56.41 -65.75 Td[(Wh)26(y)-325(is)-326(this)-327(fon)26(t)-325(size)-326(11.955)-327(in)-326(PDF?)]TJ 173.79 -564.39 Td[(1)]TJ ET Q

endstream


and you can see /F1 11.955 Tf to select the indirect font object called /F1 with size 11.955. The same effect if you use pdflatex, for example with this input:

\documentclass[12pt]{article}

\pdfcompresslevel=0
\pdfobjcompresslevel=0

\begin{document}
Why is this font size 11.955 in PDF?
\end{document}


My pdflatex (from Mac TexLive 2014) then generates:

stream
BT
/F15 11.9552 Tf 128.413 654.247 Td [(Wh)27(y)-326(is)-327(this)-326(fon)27(t)-326(size)-326(11.955)-327(in)-326(PDF?)]TJ 173.786 -564.384 Td [(1)]TJ
ET

endstream


with a font size of 11.9552.

That's due to the different idea that pdf and LaTeX have of what's a point:
• A LaTeX point is 1/72.27 inch - known in LaTeX as pt
• A pdf point is exactly 1/72 inch - known in LaTeX as bp (big point)