@Section
    @Title { Rules }
    @Tag { tbl_rule }
@Begin
@PP
There is a @Code "rule" option for drawing a rule around a cell:
@ID @OneRow @Code "@Cell rule { yes }"
Other values are {@Code no} (the default),
{@Code single} (the same as {@Code yes}), and {@Code double} (for a
double rule).
@PP
There are @Code "rulehorizontal" and @Code "rulevertical" options which
draw only horizontal or vertical rules, and also {@Code "ruleabove"},
{@Code "rulebelow"}, {@Code "ruleleft"}, and {@Code "ruleright"} options:
@ID @OneRow @Code @Verbatim {
@Tbl
    aformat { @Cell A | @Cell B }
{
@Rowa
    ruleabove { yes }
    A { Commercial property }
    B { 10% }
@Rowa
    A { Stock market }
    B { 15% }
    rulebelow { yes }
}
}
produces
@CD @OneRow @Tbl
    aformat { @Cell A | @Cell B }
{
@Rowa
    ruleabove { yes }
    A { Commercial property  }
    B { 10% }
@Rowa
    A { Stock market }
    B { 15% }
    rulebelow { yes }
}
These options take the same values as {@Code "rule"}, but draw
along only one or two of the four edges.
@PP
Other options control the appearance of rules.  Here they are with their
default values:
@ID @OneRow @Code @Verbatim {
@Tbl
    rulewidth { 0.05f }
    rulegap { 0.15f }
    rulecolour { black }
}
These say that rules are to be @Code "0.05f" wide (thick), double rules
are to appear @Code "0.15f" apart, and the colour of rules is to be black.
Once again, more specific versions of these symbols exist for controlling
above, below, left, and right rules:
@ID @OneRow @Code @Tbl
    aformat { @Cell ml { 0i } A | @Cell B | @Cell C }
    marginvertical { 0.5vx }
{
@Rowa
    A { rulehorizontalwidth }
    B { rulehorizontalgap }
    C { rulehorizontalcolour }
@Rowa
    A { ruleabovewidth }
    B { ruleabovegap }
    C { ruleabovecolour }
@Rowa
    A { rulebelowwidth }
    B { rulebelowgap }
    C { rulebelowcolour }
@Rowa
    A { ruleverticalwidth }
    B { ruleverticalgap }
    C { ruleverticalcolour }
@Rowa
    A { ruleleftwidth }
    B { ruleleftgap }
    C { ruleleftcolour }
@Rowa
    A { rulerightwidth }
    B { rulerightgap }
    C { rulerightcolour }
}
All these options have alternative, abbreviated names; and @Code { colour }
may be spelt @Code { color } wherever it appears.  Section
{@NumberOf tbl_summ} has a complete summary of all spellings of all
options.
@PP
To clarify exactly where the rules are drawn, let's start with
a cell with no rules at all:
@CD @Tbl
    mv { 0i }
    aformat { @Cell width { 3c } height { 1.2c } paint { lightgrey } A }
{
@Rowa
}
Above rules and left rules are drawn within the cell boundary, just
touching it, with any above rule overstriking any left rule:
@CD { @Tbl
    mv { 0i }
    aformat { @Cell width { 3c } height { 1.2c } paint { lightgrey } A }
{
@Rowa
}
@Background @Tbl
    mv { 0i }
    aformat { @Cell width { 3c } height { 1.2c } A }
    ruleleft { yes }
    ruleabove { yes }
    rulehorizontalwidth { 0.8v }
    ruleverticalwidth { 0.5v }
    ruleverticalcolour { grey }
    rulehorizontalcolour { black }
{
@Rowa
}
}
Below and right rules are drawn just outside the boundary of the
cell, also touching it:
@CD @Tbl mv { 0i } aformat { @Cell A | @Cell | @Cell B }
{
@Rowa
    A {
        @Tbl
            mv { 0i }
            aformat { @Cell width { 3c } height { 1.2c } paint { lightgrey } A }
        {
        @Rowa
        }
        @Background
        @Tbl
            mv { 0i }
            aformat { @Cell width { 3c } height { 1.2c } A }
            rulebelow { yes }
	    rulehorizontalwidth { 0.8v }
	    ruleverticalwidth { 0.5v }
	    ruleverticalcolour { grey }
	    rulehorizontalcolour { black }
        {
        @Rowa
        }
      }
    B {
        @Tbl
            mv { 0i }
            aformat { @Cell width { 3c } height { 1.2c } paint { lightgrey } A }
        {
        @Rowa
        }
        @Background
        @Tbl
            mv { 0i }
            aformat { @Cell width { 3c } height { 1.2c } A }
            ruleright { yes }
	    rulehorizontalwidth { 0.8v }
	    ruleverticalwidth { 0.5v }
	    ruleverticalcolour { grey }
	    rulehorizontalcolour { black }
        {
        @Rowa
        }
      }
}
@DP
When a right rule is present, any above and below rules are extended
by the width of the right rule, and they overstrike it:
@CD {
@Tbl
    mv { 0i }
    aformat { @Cell width { 3c } height { 1.2c } paint { lightgrey } A }
{
@Rowa
}
@Background
@Tbl
    mv { 0i }
    aformat { @Cell width { 3c } height { 1.2c } A }
    ruleabove { yes }
    ruleright { yes }
    rulebelow { yes }
    rulehorizontalwidth { 0.8v }
    ruleverticalwidth { 0.5v }
    ruleverticalcolour { grey }
    rulehorizontalcolour { black }
{
@Rowa
}
}
@DP
(These diagrams were produced by @Code "@Tbl" itself, using horizontal
rules of width @Code 0.8v drawn in black, and vertical rules of width
@Code 0.5v drawn in grey.)  These arrangements ensure that even thick
rules produce clean corners, and also that a right rule and a neighbouring
left rule exactly overstrike each other, as do a below rule and its
neighbouring above rule.
# @PP
# For information about rules in plain text tables, consult Section
# {@NumberOf tbl_plai}.
@End @Section