diff --git a/kwant/builder.py b/kwant/builder.py
index ff71e1a644719626c3efa1b99ed8a3e46515c187..93fcdc236e0fa6101d259fe676ffb8bd75f01b7f 100644
--- a/kwant/builder.py
+++ b/kwant/builder.py
@@ -812,7 +812,12 @@ class Builder(object):
         return chain(self.H, self.hoppings())
 
     def sites(self):
-        """Return a read-only set over all sites."""
+        """Return a read-only set over all sites.
+
+        The sites that are returned belong to the fundamental domain of the
+        `Builder` symmetry, and are not necessarily the ones that were set
+        initially (but always the equivalent ones).
+        """
         try:
             return self.H.viewkeys()
         except AttributeError:
@@ -824,7 +829,12 @@ class Builder(object):
             yield site, hvhv[1]
 
     def hoppings(self):
-        """Return an iterator over all hoppings."""
+        """Return an iterator over all Builder hoppings.
+
+        The hoppings that are returned belong to the fundamental domain of the
+        `Builder` symmetry, and are not necessarily the ones that were set
+        initially (but always the equivalent ones).
+        """
         for tail, hvhv in self.H.iteritems():
             for head, value in edges(hvhv):
                 if value is other: continue
diff --git a/kwant/lattice.py b/kwant/lattice.py
index f0b0bf4afadaf22abcf341b70ab486402ff4bc51..bc578039921227ce14b82af7712ba672b69e4877 100644
--- a/kwant/lattice.py
+++ b/kwant/lattice.py
@@ -225,6 +225,9 @@ class TranslationalSymmetry(builder.Symmetry):
     produced. A certain flexibility in choice of the fundamental domain can be
     achieved by calling manually the `add_site_group` method and providing it
     the `other_vectors` parameter.
+
+    The fundamental domain for hoppings are all hoppings ``(a, b)`` with site
+    `a` in fundamental domain of sites.
     """
     def __init__(self, periods):
         self.periods = ta.array(periods)