prawn：Prawn::Graphics::CapStyle#cap_style, #cap_style=

everyday Ruby 
#!/usr/bin/env ruby

require "prawn"

pdf = Prawn::Document.new
pdf.line_width = 20
p pdf.cap_style

y = 100
[:butt, :round, :projecting_square].each do |s|
  pdf.cap_style(s)
  pdf.move_to(100, y)
  pdf.line_to(150, y)
  pdf.line_to(150, y-50)
  pdf.stroke
  y += 100
end

pdf.render_file(File.basename($0, ".rb") + ".pdf")

で、

:butt