diff --git a/common/common.h b/common/common.h
index d731c1f..c73b853 100644
--- a/common/common.h
+++ b/common/common.h
@@ -1,7 +1,9 @@
 #ifndef __COMMON_H
 #define __COMMON_H
 
-; Using four byte quantities aabbccdd with one bit overflow/underflow, 21 bits significand, 10 bits fraction
+; Using four byte quantities aabbccdd with sign bit, overflow/underflow bit, 20 bits significand, 10 bits
+; fraction. The quantity is valid if the overflow/underflow bit agrees with the sign bit. The intent is
+; to be able to recognize an overflow/underflow situation, rescale the arguments, and repeat.
 
 ; Largest value:          $3fffffff or  1048575.999(9)
 ; Smallest value:         $c0000001 or -1048575.999(0)
@@ -132,7 +134,7 @@ _EXT_C	= $f0
 _PLS_1	= %00000100	; i.e. the $04 part of $00000400
 _MNS_1	= %11111100	; i.e. the $FC part of $FFFFFC00
 
-; masks for overflow and unerflow
+; masks for overflow and underflow
 _MSK_O	= %11000000	; mask for overflow
 _MSK_U	= %11000000	; mask for underflow