diff --git a/src/Hanafuda/Message.hs b/src/Hanafuda/Message.hs
index 4506a7f..ce33786 100644
--- a/src/Hanafuda/Message.hs
+++ b/src/Hanafuda/Message.hs
@@ -13,6 +13,7 @@ module Hanafuda.Message (
     , PublicPlayer(..)
     , PublicState(..)
     , Room
+    , orderCoordinates
   ) where
 
 import Data.Char (toLower)
@@ -153,6 +154,13 @@ instance FromJSON Coordinates
 instance ToJSON Coordinates where
   toEncoding = genericToEncoding defaultOptions
 
+orderCoordinates :: Coordinates -> Coordinates -> Maybe Ordering
+orderCoordinates coordinatesA coordinatesB
+  | gameID coordinatesA == gameID coordinatesB = Just $ compare
+    (month coordinatesA, turn coordinatesA)
+    (month coordinatesB, turn coordinatesB)
+  | otherwise = Nothing
+
 data PrivateState = PrivateState {
       link :: Coordinates
     , hands :: Map PlayerID Hanafuda.Pack