diff --git a/data-model.md b/data-model.md
index 4a72670..188e3ea 100644
--- a/data-model.md
+++ b/data-model.md
@@ -34,6 +34,10 @@ A *triple*, as in RDF, is a structure composed of three elements:
 
 *Triples* notation is `(subject, predicate, object)`.
 
+A triple `(a, b, c)` where `a`, `b` and `c` are *resources* is true if, and only if, the associated statement is true. For example `(Douglas Adams, instance of, human)` is true because *Douglas Adams is an instance of human* is a true statement but `(Douglas Adams, instance of, flower)` is false because Douglas Adams is definitively not a flower.
+
+We define also a subset of *resources* called *properties* such that `b` is a *property* if, an only if, `b` may be seen as a relation between two resources (i.e. may be used as a predicate of a meaningful triple). For example `birth date` is a *property* but `Douglas Adams` is not.
+
 We use triple formalism with some operators:
 
 ##### *full triple*
@@ -50,6 +54,13 @@ Example: a triple generated from the question “What is the birth date of Georg
 #### *list* operators
 There are some operators that manipulate lists. These operators do not preserve order.
 
+##### *inverse*
+The *inverse* is an operator of *property → property* such that `inverse(p)` is an *inverse property* of `p` i.e. a property such that for all `a`, `c`, `(a, p, c)` ↔ `(c, inverse(p), a)`. We generalize this operator on *list → list* (it returns an inverse property for each property in the input list).
+
+Example: We may have `has parent = inverse(has child)`.
+
+*inverse* operator is useful when the same statement may be written in inversed manners in the database. For example, `a` is a part of `c` may be encoded as `(a, part of, c)` or `(c, member, a)`. With *inverse*, in order to retrieve all parts of `c` we may use this single triple `(?, part of ∪ inverse(member), c) = (c, member ∪ inverse(part of), ?)`.
+
 ##### *union*
 The *union* is an operator of *list⁺ → list* that returns the union of the lists. Its notation is the infix `∪` like `l1 ∪ l2 ∪ l3`.
 
@@ -257,6 +268,28 @@ Example: the serialization of the triple `(George Washington, birth date, ?)` is
 }
 ```
 
+There is also an optional fourth attribute, `inverse-predicate` to encode triples like `(Barack Obama, residence ∪ inverse(inhabitant), ?)`:
+```
+{
+	"type": "triple",
+	"subject": {"type": "resource", "value": "Barack Obama"},
+	"predicate": {"type": "resource", "value": "residence"},
+	"inverse-predicate": {"type": "resource", "value": "inhabitant"},
+	"object": {"type": "missing"}
+}
+```
+
+Please note that the previous triple is equivalent to:
+```
+{
+    "type": "triple",
+    "subject": {"type": "missing"},
+    "predicate": {"type": "resource", "value": "inhabitant"},
+    "inverse-predicate": {"type": "resource", "value": "residence"},
+    "object": {"type": "resource", "value": "Barack Obama"}
+}
+```
+
 ### *union*, *intersection*, *and* and *or*
 There is only one parameter, *list*, that is an array containing the operator parameters.