In The TeXBook it says
The value of
\prevdepth
at the time of the\halign
is used at the beginning of the internal vertical list, and the final value of\prevdepth
is carried to the enclosing vertical list when the alignment is completed, so that the interline glue is calculated properly at the beginning and end of the alignment.
In section 15.2 of TeX by Topic, it says
At the beginning of a vertical list
\prevdepth
is set to-1000pt
, except in an\halign
and\noalign
code contained therein, where it is carried over from the surrounding list.
I'm able to understand the sentence in The TeXBook, but could not understand the sentence in TeX by Topic. Could someone give some examples about \prevdepth
and \noalign
?