diff --git a/skimage/graph/_ncut.py b/skimage/graph/_ncut.py
index e31b9431..3e961ad4 100644
--- a/skimage/graph/_ncut.py
+++ b/skimage/graph/_ncut.py
@@ -4,7 +4,23 @@ from scipy import sparse
 
 
 def DW_matrix(graph):
+    """Returns the diagonal and weight matrix of a graph.
 
+    Parameters
+    ----------
+    graph : RAG
+        A Region Adjacency Graph.
+
+    Returns
+    -------
+    D : csc_matrix
+        The diagonal matrix of the graph. `D[i,i]` is the sum of weights of all
+        edges incident on `i`. All other enteries are `0`.
+    W : csc_matrix
+        The weight matrix of the graph. `W[i,j]` is the weight of the edge
+        joining `i` to `j`.
+    """
+    #Cause sparse.eigsh prefers CSC format
     W = nx.to_scipy_sparse_matrix(graph, format='csc')
     entries = W.sum(0)
     D = sparse.dia_matrix((entries, 0), shape=W.shape).tocsc()
@@ -12,7 +28,23 @@ def DW_matrix(graph):
 
 
 def ncut_cost(mask, D, W):
+    """Returns the N-cut cost of a bi-partition of a graph.
 
+    Parameters
+    ----------
+    mask : ndarray
+        The mask for the nodes in the graph. Nodes corrsesponding to a `True`
+        value are in one set.
+    D : csc_matrix
+        The diagonal matrix of the graph.
+    W : csc_matrix
+        The weight matrix of the graph.
+
+    Returns
+    -------
+    cost : float
+        The cost of performing the N-cut.
+    """
     mask = np.array(mask)
     mask_list = [np.logical_xor(mask[i], mask) for i in range(mask.shape[0])]
     mask_array = np.array(mask_list)
@@ -25,6 +57,18 @@ def ncut_cost(mask, D, W):
 
 
 def normalize(a):
+    """Normalize values in an array between `0` and `1`.
+
+    Parameters
+    ----------
+    a : ndarray
+        The array to be normalized.
+
+    Returns
+    -------
+    out : ndarray
+        The normalized array.
+    """
     mi = a.min()
     mx = a.max()
     return (a - mi) / (mx - mi)