diff --git a/motile/expressions.py b/motile/expressions.py
index 774cf77..b41c5b7 100644
--- a/motile/expressions.py
+++ b/motile/expressions.py
@@ -11,18 +11,24 @@
 
 
 class VariableType(IntEnum):
+    """Type of a variable."""
+
     Continuous = 0
     Integer = auto()
     Binary = auto()
 
 
 class Relation(IntEnum):
+    """Type of a constraint relation."""
+
     LessEqual = 0
     Equal = auto()
     GreaterEqual = auto()
 
 
 class Sense(IntEnum):
+    """Type of an objective sense."""
+
     Minimize = 0
     Maximize = auto()