diff --git a/sys/sys/priority.h b/sys/sys/priority.h
--- a/sys/sys/priority.h
+++ b/sys/sys/priority.h
@@ -64,17 +64,23 @@
  */
 
 /*
- * Priorities range from 0 to 255, but differences of less then 4 (RQ_PPQ)
- * are insignificant.  Ranges are as follows:
+ * Priorities range from 0 to 255.  Ranges are as follows:
  *
- * Interrupt threads:		0 - 15
- * Realtime user threads:	16 - 47
- * Top half kernel threads:	48 - 87
- * Time sharing user threads:	88 - 223
+ * Interrupt threads:		0 - 7
+ * Realtime user threads:	8 - 39
+ * Top half kernel threads:	40 - 55
+ * Time sharing user threads:	56 - 223
  * Idle user threads:		224 - 255
  *
- * XXX If/When the specific interrupt thread and top half thread ranges
- * disappear, a larger range can be used for user processes.
+ * Priority levels of rtprio(2)'s RTP_PRIO_FIFO and RTP_PRIO_REALTIME and
+ * POSIX's SCHED_FIFO and SCHED_RR are directly mapped to the internal realtime
+ * range mentioned above by a simple translation.  This range's length
+ * consequently cannot be changed without impacts on the scheduling priority
+ * code, and in any case must never be smaller than 32 for POSIX compliance and
+ * rtprio(2) backwards compatibility.  Similarly, priority levels of rtprio(2)'s
+ * RTP_PRIO_IDLE are directly mapped to the internal idle range above (and,
+ * soon, those of the to-be-introduced SCHED_IDLE policy as well), so changing
+ * that range is subject to the same caveats and restrictions.
  */
 
 #define	PRI_MIN			(0)		/* Highest priority. */
@@ -88,34 +94,34 @@
  * decay to lower priorities if they run for full time slices.
  */
 #define	PI_REALTIME		(PRI_MIN_ITHD + 0)
-#define	PI_INTR			(PRI_MIN_ITHD + 4)
+#define	PI_INTR			(PRI_MIN_ITHD + 1)
 #define	PI_AV			PI_INTR
 #define	PI_NET			PI_INTR
 #define	PI_DISK			PI_INTR
 #define	PI_TTY			PI_INTR
 #define	PI_DULL			PI_INTR
-#define	PI_SOFT			(PRI_MIN_ITHD + 8)
+#define	PI_SOFT			(PRI_MIN_ITHD + 2)
 #define	PI_SOFTCLOCK		PI_SOFT
 #define	PI_SWI(x)		PI_SOFT
 
-#define	PRI_MIN_REALTIME	(16)
+#define	PRI_MIN_REALTIME	(8)
 #define	PRI_MAX_REALTIME	(PRI_MIN_KERN - 1)
 
-#define	PRI_MIN_KERN		(48)
+#define	PRI_MIN_KERN		(40)
 #define	PRI_MAX_KERN		(PRI_MIN_TIMESHARE - 1)
 
 #define	PSWP			(PRI_MIN_KERN + 0)
-#define	PVM			(PRI_MIN_KERN + 4)
-#define	PINOD			(PRI_MIN_KERN + 8)
-#define	PRIBIO			(PRI_MIN_KERN + 12)
-#define	PVFS			(PRI_MIN_KERN + 16)
-#define	PZERO			(PRI_MIN_KERN + 20)
-#define	PSOCK			(PRI_MIN_KERN + 24)
-#define	PWAIT			(PRI_MIN_KERN + 28)
-#define	PLOCK			(PRI_MIN_KERN + 32)
-#define	PPAUSE			(PRI_MIN_KERN + 36)
+#define	PVM			(PRI_MIN_KERN + 1)
+#define	PINOD			(PRI_MIN_KERN + 2)
+#define	PRIBIO			(PRI_MIN_KERN + 3)
+#define	PVFS			(PRI_MIN_KERN + 4)
+#define	PZERO			(PRI_MIN_KERN + 5)
+#define	PSOCK			(PRI_MIN_KERN + 6)
+#define	PWAIT			(PRI_MIN_KERN + 7)
+#define	PLOCK			(PRI_MIN_KERN + 8)
+#define	PPAUSE			(PRI_MIN_KERN + 9)
 
-#define	PRI_MIN_TIMESHARE	(88)
+#define	PRI_MIN_TIMESHARE	(56)
 #define	PRI_MAX_TIMESHARE	(PRI_MIN_IDLE - 1)
 
 #define	PUSER			(PRI_MIN_TIMESHARE)