.PS
.ps 20
boxwid	=	2*boxwid
define boxit @
	box "$1" with .n at ($2.s.x, $2.s.y-lineht/2)
	arrow from last box.n to $2.s
@
## figure 4
B1:	box "APPLICATION"
B2:	boxit(PRESENTATION,B1)
B3:	boxit(SESSION,B2)
B4:	box "TRANSPORT" with .n at (B3.s.x, B3.s.y-lineht)
	arrow from B4.n to B3.s
B5:	boxit(NETWORK,B4)
##
B8:	box with .n at (B1.n.x+boxwid+linewid, B1.n.y) height (B1.n.y-B3.s.y) \
	"APPLICATION"
B9:	box "TRANSPORT" with .n at (B8.s.x, B4.n.y)
	arrow from B9.n to B8.s
B10:	boxit(NETWORK,B9)
##
	move to (B5.s.x, B5.s.y-lineht); "ISO stack"
	move to (B10.s.x, B10.s.y-lineht); "DDN stack"
##
.PE