diff --git a/lib/net/imap/sequence_set.rb b/lib/net/imap/sequence_set.rb
index 1d09aa05..a8091c05 100644
--- a/lib/net/imap/sequence_set.rb
+++ b/lib/net/imap/sequence_set.rb
@@ -183,11 +183,15 @@ class IMAP
     # - #max: Returns the maximum number in the set.
     # - #minmax: Returns the minimum and maximum numbers in the set.
     #
-    # <i>Accessing value by (normalized) offset:</i>
+    # <i>Accessing value by offset in sorted set:</i>
     # - #[] (aliased as #slice): Returns the number or consecutive subset at a
-    #   given offset or range of offsets.
-    # - #at: Returns the number at a given offset.
-    # - #find_index: Returns the given number's offset in the set
+    #   given offset or range of offsets in the sorted set.
+    # - #at: Returns the number at a given offset in the sorted set.
+    # - #find_index: Returns the given number's offset in the sorted set.
+    #
+    # <i>Accessing value by offset in ordered entries</i>
+    # - #find_ordered_index: Returns the index of the given number's first
+    #   occurrence in entries.
     #
     # <i>Set cardinality:</i>
     # - #count (aliased as #size): Returns the count of numbers in the set.
@@ -1083,33 +1087,48 @@ def has_duplicates?
         count_with_duplicates != count
       end
 
-      # Returns the index of +number+ in the set, or +nil+ if +number+ isn't in
-      # the set.
+      # Returns the (sorted and deduplicated) index of +number+ in the set, or
+      # +nil+ if +number+ isn't in the set.
       #
-      # Related: #[]
+      # Related: #[], #at, #find_ordered_index
       def find_index(number)
         number = to_tuple_int number
-        each_tuple_with_index do |min, max, idx_min|
+        each_tuple_with_index(@tuples) do |min, max, idx_min|
           number <  min and return nil
           number <= max and return from_tuple_int(idx_min + (number - min))
         end
         nil
       end
 
+      # Returns the first index of +number+ in the ordered #entries, or
+      # +nil+ if +number+ isn't in the set.
+      #
+      # Related: #find_index
+      def find_ordered_index(number)
+        number = to_tuple_int number
+        each_tuple_with_index(each_entry_tuple) do |min, max, idx_min|
+          if min <= number && number <= max
+            return from_tuple_int(idx_min + (number - min))
+          end
+        end
+        nil
+      end
+
       private
 
-      def each_tuple_with_index
+      def each_tuple_with_index(tuples)
         idx_min = 0
-        @tuples.each do |min, max|
-          yield min, max, idx_min, (idx_max = idx_min + (max - min))
+        tuples.each do |min, max|
+          idx_max = idx_min + (max - min)
+          yield min, max, idx_min, idx_max
           idx_min = idx_max + 1
         end
         idx_min
       end
 
-      def reverse_each_tuple_with_index
+      def reverse_each_tuple_with_index(tuples)
         idx_max = -1
-        @tuples.reverse_each do |min, max|
+        tuples.reverse_each do |min, max|
           yield min, max, (idx_min = idx_max - (max - min)), idx_max
           idx_max = idx_min - 1
         end
@@ -1120,18 +1139,21 @@ def reverse_each_tuple_with_index
 
       # :call-seq: at(index) -> integer or nil
       #
-      # Returns a number from +self+, without modifying the set.  Behaves the
-      # same as #[], except that #at only allows a single integer argument.
+      # Returns the number at the given +index+ in the sorted set, without
+      # modifying the set.
+      #
+      # +index+ is interpreted the same as in #[], except that #at only allows a
+      # single integer argument.
       #
       # Related: #[], #slice
       def at(index)
         index = Integer(index.to_int)
         if index.negative?
-          reverse_each_tuple_with_index do |min, max, idx_min, idx_max|
+          reverse_each_tuple_with_index(@tuples) do |min, max, idx_min, idx_max|
             idx_min <= index and return from_tuple_int(min + (index - idx_min))
           end
         else
-          each_tuple_with_index do |min, _, idx_min, idx_max|
+          each_tuple_with_index(@tuples) do |min, _, idx_min, idx_max|
             index <= idx_max and return from_tuple_int(min + (index - idx_min))
           end
         end
@@ -1146,17 +1168,18 @@ def at(index)
       #    seqset[range]         -> sequence set or nil
       #    slice(range)          -> sequence set or nil
       #
-      # Returns a number or a subset from +self+, without modifying the set.
+      # Returns a number or a subset from the _sorted_ set, without modifying
+      # the set.
       #
       # When an Integer argument +index+ is given, the number at offset +index+
-      # is returned:
+      # in the sorted set is returned:
       #
       #     set = Net::IMAP::SequenceSet["10:15,20:23,26"]
       #     set[0]   #=> 10
       #     set[5]   #=> 15
       #     set[10]  #=> 26
       #
-      # If +index+ is negative, it counts relative to the end of +self+:
+      # If +index+ is negative, it counts relative to the end of the sorted set:
       #     set = Net::IMAP::SequenceSet["10:15,20:23,26"]
       #     set[-1]  #=> 26
       #     set[-3]  #=> 22
@@ -1168,13 +1191,14 @@ def at(index)
       #     set[11]  #=> nil
       #     set[-12] #=> nil
       #
-      # The result is based on the normalized set—sorted and de-duplicated—not
-      # on the assigned value of #string.
+      # The result is based on the sorted and de-duplicated set, not on the
+      # ordered #entries in #string.
       #
       #     set = Net::IMAP::SequenceSet["12,20:23,11:16,21"]
       #     set[0]   #=> 11
       #     set[-1]  #=> 23
       #
+      # Related: #at
       def [](index, length = nil)
         if    length              then slice_length(index, length)
         elsif index.is_a?(Range)  then slice_range(index)
diff --git a/test/net/imap/test_sequence_set.rb b/test/net/imap/test_sequence_set.rb
index b6995305..58aefae5 100644
--- a/test/net/imap/test_sequence_set.rb
+++ b/test/net/imap/test_sequence_set.rb
@@ -251,6 +251,44 @@ def obj.to_sequence_set; 192_168.001_255 end
     assert_equal 2**32 - 1, SequenceSet.full.find_index(:*)
   end
 
+  test "#find_ordered_index" do
+    assert_equal         9, SequenceSet.full.find_ordered_index(10)
+    assert_equal        99, SequenceSet.full.find_ordered_index(100)
+    assert_equal 2**32 - 1, SequenceSet.full.find_ordered_index(:*)
+    assert_nil SequenceSet.empty.find_index(1)
+    set = SequenceSet["9,8,7,6,5,4,3,2,1"]
+    assert_equal 0, set.find_ordered_index(9)
+    assert_equal 1, set.find_ordered_index(8)
+    assert_equal 2, set.find_ordered_index(7)
+    assert_equal 3, set.find_ordered_index(6)
+    assert_equal 4, set.find_ordered_index(5)
+    assert_equal 5, set.find_ordered_index(4)
+    assert_equal 6, set.find_ordered_index(3)
+    assert_equal 7, set.find_ordered_index(2)
+    assert_equal 8, set.find_ordered_index(1)
+    assert_nil      set.find_ordered_index(10)
+    set = SequenceSet["7:9,5:6"]
+    assert_equal 0, set.find_ordered_index(7)
+    assert_equal 1, set.find_ordered_index(8)
+    assert_equal 2, set.find_ordered_index(9)
+    assert_equal 3, set.find_ordered_index(5)
+    assert_equal 4, set.find_ordered_index(6)
+    assert_nil   set.find_ordered_index(4)
+    set = SequenceSet["1000:1111,1:100"]
+    assert_equal   0, set.find_ordered_index(1000)
+    assert_equal 100, set.find_ordered_index(1100)
+    assert_equal 112, set.find_ordered_index(1)
+    assert_equal 121, set.find_ordered_index(10)
+    set = SequenceSet["1,1,1,1,51,50,4,11"]
+    assert_equal   0, set.find_ordered_index(1)
+    assert_equal   4, set.find_ordered_index(51)
+    assert_equal   5, set.find_ordered_index(50)
+    assert_equal   6, set.find_ordered_index(4)
+    assert_equal   7, set.find_ordered_index(11)
+    assert_equal   1, SequenceSet["1,*"].find_ordered_index(-1)
+    assert_equal   0, SequenceSet["*,1"].find_ordered_index(-1)
+  end
+
   test "#limit" do
     set = SequenceSet["1:100,500"]
     assert_equal [1..99],               set.limit(max: 99).ranges