diff --git a/Utils/Conventions.ml b/Utils/Conventions.ml
new file mode 100644
index 0000000..e70918c
--- /dev/null
+++ b/Utils/Conventions.ml
@@ -0,0 +1,30 @@
+(** Contains the conventions relative to the program.
+
+  The phase convention is given by:
+  {% $\sum_i c_i > 0$ %} or {% $\min_i c_i \ge  0$ %}
+
+*)
+
+open Lacaml.D
+
+let in_phase vec =
+  let s = Vec.sum vec in
+  if s = 0. then 
+    let rec first_non_zero k =
+      if k > Vec.dim vec then
+        k-1
+      else if vec.{k} = 0. then
+        first_non_zero (k+1)
+      else k
+    in
+    let k = first_non_zero 1 in
+    vec.{k} >= 0.
+  else
+    s > 0.
+
+
+let rephase mat  =
+  Mat.to_col_vecs mat
+  |> Array.map (fun v -> if in_phase v then v else Vec.neg v)
+  |> Mat.of_col_vecs
+
diff --git a/Utils/Conventions.mli b/Utils/Conventions.mli
new file mode 100644
index 0000000..e32613f
--- /dev/null
+++ b/Utils/Conventions.mli
@@ -0,0 +1,16 @@
+(** Contains the conventions relative to the program.
+
+  The phase convention is given by:
+  {% $\sum_i c_i > 0$ %} or {% $\min_i c_i \ge  0$ %}
+
+*)
+
+open Lacaml.D
+
+
+val in_phase : Vec.t -> bool
+(** Checks if one MO respects the phase convention *)
+
+val rephase : Mat.t -> Mat.t
+(** Apply the phase convention to the MOs. *)
+