Equivalent Leaders construction

Question

What is the equivalent construction of

\leaders\hrule height 1pt depth 1 pt\hskip 20pt \relax

in terms of the following construction or something similar

\leaders\hbox{\vrule height 1pt depth 1pt}\hskip 20pt \relax

In the first construction, there is no white gaps between the \hrules but in the second construction, there can be white gaps (horizontally) between the \vrules.

Answer 1

The first case is \leaders with a rule. The rule can be easily extended to the desired width. The result is just one rule with the specified leaders width.

The second case is a box with fixed dimensions. The width of the box is the default rule width 0.4 pt. Then the space to be filled is filled with the boxes. There are three variants:

• \leaders sets the boxes at fixed grid position. It is intended for dots in the table of contents, for example. Then no matter, where the dots start, they are always vertically aligned.

• \xleaders calculates, how many boxes are needed to fill the space without overfilling. Then the space is divided in equal sized segments and the box is horizontally centered. The example below fills the space 20 pt with a box of width 7 pt. Thus two boxes fit in the space and the result is a "dashed" line.

• \cleaders. Since the "dash" effect of the previous version is sometimes not desirable, \cleaders moves the boxes in the middle and put the remaining unfilled space at both sides of the space to be filled.

Example:

\noindent
\leaders\hrule height 1pt depth 1 pt\hskip 20pt \relax
[rule]

\noindent
\leaders\hbox{\vrule height 1pt depth 1pt width 7pt}\hskip 20pt \relax
[leaders + box]

\noindent
\xleaders\hbox{\vrule height 1pt depth 1pt width 7pt}\hskip 20pt \relax
[xleaders + box]

\noindent
\cleaders\hbox{\vrule height 1pt depth 1pt width 7pt}\hskip 20pt \relax
[cleaders + box]

\bye