The number of spaces used for the right margin. The value provided is silently converted to a positive integer value.

                           columns
<-------------------------------------------------------------->
<-----------><------><---------------------------><------------>
 left margin  indent  text is formatted into here  RIGHT MARGIN

Source Code

# File action_mailer/vendor/text-format-0.6.3/text/format.rb, line 241
def right_margin=(r)
  @right_margin = posint(r)
end