diff --git a/include/ruby/internal/arithmetic/fixnum.h b/include/ruby/internal/arithmetic/fixnum.h
index 8a3354a377..c8927ac824 100644
--- a/include/ruby/internal/arithmetic/fixnum.h
+++ b/include/ruby/internal/arithmetic/fixnum.h
@@ -22,23 +22,39 @@
  */
 #include "ruby/backward/2/limits.h"
 
-#define FIXABLE    RB_FIXABLE
-#define FIXNUM_MAX RUBY_FIXNUM_MAX
-#define FIXNUM_MIN RUBY_FIXNUM_MIN
-#define NEGFIXABLE RB_NEGFIXABLE
-#define POSFIXABLE RB_POSFIXABLE
+#define FIXABLE    RB_FIXABLE      /**< @old{RB_FIXABLE} */
+#define FIXNUM_MAX RUBY_FIXNUM_MAX /**< @old{RUBY_FIXNUM_MAX} */
+#define FIXNUM_MIN RUBY_FIXNUM_MIN /**< @old{RUBY_FIXNUM_MIN} */
+#define NEGFIXABLE RB_NEGFIXABLE   /**< @old{RB_NEGFIXABLE} */
+#define POSFIXABLE RB_POSFIXABLE   /**< @old{RB_POSFIXABLE} */
 
-/*
+/**
+ * Checks if the passed value is in  range of fixnum, assuming it is a positive
+ * number.  Can sometimes be useful for C's unsigned integer types.
+ *
+ * @internal
+ *
  * FIXABLE can be applied to anything, from double to intmax_t.  The problem is
  * double.   On a  64bit system  RUBY_FIXNUM_MAX is  4,611,686,018,427,387,903,
  * which is not representable by a double.  The nearest value that a double can
  * represent  is   4,611,686,018,427,387,904,  which   is  not   fixable.   The
- * seemingly-stragne "< FIXNUM_MAX + 1" expression below is due to this.
+ * seemingly-strange "< FIXNUM_MAX + 1" expression below is due to this.
  */
 #define RB_POSFIXABLE(_) ((_) <  RUBY_FIXNUM_MAX + 1)
+
+/**
+ * Checks if the passed value is in  range of fixnum, assuming it is a negative
+ * number.  This is an implementation of #RB_FIXABLE.  Rarely used stand alone.
+ */
 #define RB_NEGFIXABLE(_) ((_) >= RUBY_FIXNUM_MIN)
+
+/** Checks if the passed value is in  range of fixnum */
 #define RB_FIXABLE(_)    (RB_POSFIXABLE(_) && RB_NEGFIXABLE(_))
+
+/** Maximum possible value that a fixnum can represent. */
 #define RUBY_FIXNUM_MAX  (LONG_MAX / 2)
+
+/** Minimum possible value that a fixnum can represent. */
 #define RUBY_FIXNUM_MIN  (LONG_MIN / 2)
 
 #endif /* RBIMPL_ARITHMETIC_FIXNUM_H */