- (thick-thickness ,number? "Bar line thickness, measured in
-@code{line-thickness}.")
- (thickness ,number? "Line thickness, generally measured in
-@code{line-thickness}.")
+ (thick-thickness ,number? "Thickness of the thick line in a
+bar line, expressed as a multiple of the default staff-line
+thickness (i.e. the visual output is @emph{not} influenced by
+changes to @code{@var{Staff}.StaffSymbol.thickness}).")
+ (thickness ,number? "For grobs made up of lines, this is the
+thickness of the line. For slurs and ties, this is the distance
+between the two arcs of the curve's outline at its thickest point,
+not counting the diameter of the virtual @qq{pen} that draws the
+arcs. This property is expressed as a multiple of the current
+staff-line thickness (i.e. the visual output is influenced by
+changes to @code{@var{Staff}.StaffSymbol.thickness}).")