# Why \nointerlineskip if background of Beamer-color frametitle is empty?

I'm trying to read the source of Beamer default outer theme (the file `beamerouterthemedefault.sty`). In line 144 there is this code:

``````\defbeamertemplate*{frametitle}{default}[1][left]
{
\ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
% more things here
}
``````

My question is simple: what is the purpose of this `\nointerlineskip`, and how is this connected to the background of the `frametitle` Beamer-color?

Edit: I checked on the repository on Bitbucket, and couldn't find any hints there (I hoped for some enlightening commit message found through "blame" - but no, nothing.)

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`\nointerlineskip` is useful, for example, when one wants to place two boxes (in vertical mode) one exactly above the other, with no glue between them. –  egreg Mar 13 '13 at 9:52
@egreg: I know that, but the frametitle seems to be at the very top of the page anyway. After asking the question, I gave it a little thinking, and I have two hypotheses: (1) maybe `\nointerlineskip` suppresses `\topskip`? (I don't think so.) (2) maybe `beamercolorbox` puts some kind of a box above its contents (probably yes, though it'll take some time for me to analyse its code). –  mbork Mar 13 '13 at 10:00
After skimming through beamer sources, it seems that hypothesis (2) is very probable. If nobody wants to answer this question, I'll probably be able to answer it myself in some time;)... –  mbork Mar 13 '13 at 10:06
@mbork Care to answer here? (Till left us basically no notes on lots of odd things in `beamer`, and that makes my life 'interesting' whenever I try to fix something.) –  Joseph Wright Aug 10 '13 at 9:23