What is the function of \offinterlineskip

Question

Please excuse me if this is silly question...

Can any one explain the function of \offinterlineskip? And where should be applicable?

Answer 1

\offinterlineskip is a macro defined as

\def\offinterlineskip{%
  \baselineskip=-1000pt
  \lineskip=0pt
  \lineskiplimit=\maxdimen
}

so within its scope TeX makes no attempt to make even line spacing and just stacks things vertically.

Compare the normal setting on the right where a line of . gets the same space as a line of A, with the setting with \offinterlineskip

\documentclass{article}

\begin{document}

\parbox[t]{3cm}{\offinterlineskip
A A A A A A A A A A A A 
A A A A A A A A A A A A 
A A A A A A A A A A A A 
x x x x x x x x x x x x 
x x x x x x x x x x x x 
x x x x x x x x x x x x 
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
}
\parbox[t]{3cm}{
A A A A A A A A A A A A 
A A A A A A A A A A A A 
A A A A A A A A A A A A 
x x x x x x x x x x x x 
x x x x x x x x x x x x 
x x x x x x x x x x x x 
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
}

\end{document}

Obviously this looks fairly horrible if used for paragraphs of text as here but it can be useful in special situations where you have constructed boxes with specific contents that want to be stacked exactly without TeX inserting any additional inter-line space.