digraph shells {
	size="7,8";
	node [fontsize=24, shape = plaintext];

	1972 -> 1976;
	1976 -> 1978;
	1978 -> 1980;
	1980 -> 1982;
	1982 -> 1984;
	1984 -> 1986;
	1986 -> 1988;
	1988 -> 1990;
	1990 -> future;

	node [fontsize=20, shape = box];
	{ rank=same;  1976 Mashey Bourne; }
	{ rank=same;  1978 Formshell csh; }
	{ rank=same;  1980 esh vsh; }
	{ rank=same;  1982 ksh "System-V"; }
	{ rank=same;  1984 v9sh tcsh; }
	{ rank=same;  1986 "ksh-i"; }
	{ rank=same;  1988 KornShell Perl rc; }
	{ rank=same;  1990 tcl Bash; }
	{ rank=same;  "future" POSIX "ksh-POSIX"; }

	Thompson -> Mashey;
	Thompson -> Bourne;
	Thompson -> csh;
	csh -> tcsh;
	Bourne -> ksh;
	Bourne -> esh;
	Bourne -> vsh;
	Bourne -> "System-V";
	Bourne -> v9sh;
	v9sh -> rc;
	Bourne -> Bash;
	"ksh-i" -> Bash;
	KornShell -> Bash;
	esh -> ksh;
	vsh -> ksh;
	Formshell -> ksh;
	csh -> ksh;
	KornShell -> POSIX;
	"System-V" -> POSIX;
	ksh -> "ksh-i";
	"ksh-i" -> KornShell;
	KornShell -> "ksh-POSIX";
	Bourne -> Formshell;

	edge [style=invis];
	1984 -> v9sh -> tcsh ;
	1988 -> rc -> KornShell;
	Formshell -> csh;
	KornShell -> Perl;
}