diff --git a/commands/sh/sh3.c b/commands/sh/sh3.c
index 80df511d3..cdba82b58 100644
--- a/commands/sh/sh3.c
+++ b/commands/sh/sh3.c
@@ -854,7 +854,7 @@ register struct op *t;
 	register int  resetsig;
 
 	if (t->words[1] == NULL) {
-		for (i=0; i<=_NSIG; i++)
+		for (i=0; i<_NSIG; i++)
 			if (trap[i]) {
 				prn(i);
 				prs(": ");
@@ -894,7 +894,7 @@ char *s;
 {
 	register int n;
 
-	if ((n = getn(s)) < 0 || n > _NSIG) {
+	if ((n = getn(s)) < 0 || n >= _NSIG) {
 		err("trap: bad signal number");
 		n = 0;
 	}
diff --git a/commands/sh/sh4.c b/commands/sh/sh4.c
index 2603abbd6..4fe04df73 100644
--- a/commands/sh/sh4.c
+++ b/commands/sh/sh4.c
@@ -387,7 +387,7 @@ int quoted;
 	}
 	*e.linep = 0;
 	/* allow trapped signals */
-	for (i=0; i<=_NSIG; i++)
+	for (i=0; i<_NSIG; i++)
 		if (ourtrap[i] && signal(i, SIG_IGN) != SIG_IGN)
 			signal(i, SIG_DFL);
 	dup2(pf[1], 1);
diff --git a/commands/simple/kill.c b/commands/simple/kill.c
index 7f25e4eef..354287f87 100644
--- a/commands/simple/kill.c
+++ b/commands/simple/kill.c
@@ -69,7 +69,7 @@ char **argv;
 	}
 	if (sig < 0) {						/* numeric? */
 		ul = strtoul(argv[1] + 1, &end, 10);
-		if (end == argv[1] + 1 || *end != 0 || ul > _NSIG) usage();
+		if (end == argv[1] + 1 || *end != 0 || ul >= _NSIG) usage();
 		sig = ul;
 	}
 	argv++;
diff --git a/commands/simple/login.c b/commands/simple/login.c
index c5cf560ca..1eed3ae37 100644
--- a/commands/simple/login.c
+++ b/commands/simple/login.c
@@ -445,7 +445,7 @@ char *argv[];
 
 	/* Reset signals to default values. */
 	sa.sa_handler = SIG_DFL;
-	for (n = 1; n <= _NSIG; ++n) sigaction(n, &sa, NULL);
+	for (n = 1; n < _NSIG; ++n) sigaction(n, &sa, NULL);
 
 	/* Execute the user's shell. */
 	execve(sh, argx, env);
diff --git a/kernel/system.c b/kernel/system.c
index e0ee6fbd5..b949ac2db 100644
--- a/kernel/system.c
+++ b/kernel/system.c
@@ -336,7 +336,7 @@ PUBLIC void send_sig(int proc_nr, int sig_nr)
  *===========================================================================*/
 PUBLIC void cause_sig(proc_nr, sig_nr)
 int proc_nr;			/* process to be signalled */
-int sig_nr;			/* signal to be sent, 1 to _NSIG */
+int sig_nr;			/* signal to be sent */
 {
 /* A system process wants to send a signal to a process.  Examples are:
  *  - HARDWARE wanting to cause a SIGSEGV after a CPU exception
diff --git a/kernel/system/do_kill.c b/kernel/system/do_kill.c
index f5d1d8685..118a53773 100644
--- a/kernel/system/do_kill.c
+++ b/kernel/system/do_kill.c
@@ -32,7 +32,7 @@ message *m_ptr;			/* pointer to request message */
   proc_nr_e= m_ptr->SIG_ENDPT;
 
   if (!isokendpt(proc_nr_e, &proc_nr)) return(EINVAL);
-  if (sig_nr > _NSIG) return(EINVAL);
+  if (sig_nr >= _NSIG) return(EINVAL);
   if (iskerneln(proc_nr)) return(EPERM);
 
   /* Set pending signal to be processed by the PM. */
diff --git a/lib/ansi/raise.c b/lib/ansi/raise.c
index fb444291a..a7e951c6e 100644
--- a/lib/ansi/raise.c
+++ b/lib/ansi/raise.c
@@ -15,7 +15,7 @@ pid_t _getpid(void);
 int
 raise(int sig)
 {
-	if (sig < 0 || sig > _NSIG)
+	if (sig < 0 || sig >= _NSIG)
 		return -1;
 	return _kill(_getpid(), sig);
 }
diff --git a/lib/ansi/signal.c b/lib/ansi/signal.c
index bae9b5290..ff476e0b9 100644
--- a/lib/ansi/signal.c
+++ b/lib/ansi/signal.c
@@ -11,7 +11,7 @@ sighandler_t disp;		/* signal handler, or SIG_DFL, or SIG_IGN */
 {
   struct sigaction sa, osa;
 
-  if (sig <= 0 || sig > _NSIG || sig == SIGKILL) {
+  if (sig <= 0 || sig >= _NSIG || sig == SIGKILL) {
 	errno = EINVAL;
 	return(SIG_ERR);
   }
diff --git a/lib/posix/_sigset.c b/lib/posix/_sigset.c
index 91226de5f..a1813cf93 100644
--- a/lib/posix/_sigset.c
+++ b/lib/posix/_sigset.c
@@ -2,6 +2,8 @@
 /* System processes use simpler macros with no range error checking (defined in
  * signal.h). The ANSI signal() implementation now also uses the macro
  * versions, which makes hiding of the functions here a historical remains.
+ *
+ * _NSIG is supposed to be the highest signal number plus one.
  */
 #define sigaddset	_sigaddset
 #define sigdelset	_sigdelset
@@ -13,10 +15,10 @@
 /* Low bit of signal masks. */
 #define SIGBIT_0	((sigset_t) 1)
 
-/* Mask of valid signals (0 - _NSIG). */
-#define SIGMASK		(((SIGBIT_0 << _NSIG) << 1) - 1)
+/* Mask of valid signals (0 - (_NSIG-1)). */
+#define SIGMASK		((SIGBIT_0 << _NSIG) - 1)
 
-#define sigisvalid(signo) ((unsigned) (signo) <= _NSIG)
+#define sigisvalid(signo) ((unsigned) (signo) < _NSIG)
 
 PUBLIC int sigaddset(set, signo)
 sigset_t *set;
diff --git a/test/test37.c b/test/test37.c
index d6e5c6f28..0193db288 100644
--- a/test/test37.c
+++ b/test/test37.c
@@ -238,10 +238,8 @@ void test37b()
   if (sigdelset(&s_nokill, SIGKILL) != 0) e(8);
   s_nokill_stop = s_nokill;
   if (sigdelset(&s_nokill_stop, SIGSTOP) != 0) e(8);
-#ifndef _MINIX /* XXX - should unsupported signals be <= _NSIG? */
-  if (SIGSTOP > _NSIG) e(666);
-  if (SIGSTOP <= _NSIG && sigdelset(&s_nokill, SIGSTOP) != 0) e(888);
-#endif /* _MINIX */
+  if (SIGSTOP >= _NSIG) e(666);
+  if (SIGSTOP < _NSIG && sigdelset(&s_nokill, SIGSTOP) != 0) e(888);
 
   /* Now get most of the signals into default state.  Don't change SIGINT
   * or SIGQUIT, so this program can be killed.  SIGKILL is also special.
@@ -427,7 +425,7 @@ void test37c()
   if (signal(SIGINT, catch1) != SIG_DFL) e(11);
 
   /* Verify that SIG_ERR is correctly generated. */
-  if (signal(_NSIG + 1, catch1) != SIG_ERR) e(12);
+  if (signal(_NSIG, catch1) != SIG_ERR) e(12);
   if (signal(0, catch1) != SIG_ERR) e(13);
   if (signal(-1, SIG_DFL) != SIG_ERR) e(14);
 
@@ -987,8 +985,8 @@ void clearsigstate()
   sigset_t sigset_var;
 
   /* Clear the signal state. */
-  for (i = 1; i <= _NSIG; i++) signal(i, SIG_IGN);
-  for (i = 1; i <= _NSIG; i++) signal(i, SIG_DFL);
+  for (i = 1; i < _NSIG; i++) signal(i, SIG_IGN);
+  for (i = 1; i < _NSIG; i++) signal(i, SIG_DFL);
   sigfillset(&sigset_var);
   sigprocmask(SIG_UNBLOCK, &sigset_var, (sigset_t *)NULL);
 }