diff --git a/src/pint/pint_matrix.py b/src/pint/pint_matrix.py
index 3ae2ae2dc..89b1e1a63 100644
--- a/src/pint/pint_matrix.py
+++ b/src/pint/pint_matrix.py
@@ -1,6 +1,7 @@
 """ pint_matrix module defines the pint matrix base class, the design matrix .  and the covariance matrix
 """
 
+from typing import List, Optional
 import numpy as np
 from itertools import combinations
 import astropy.units as u
@@ -41,7 +42,7 @@ class PintMatrix:
     TODO: 1. add index to label mapping
     """
 
-    def __init__(self, matrix, axis_labels):
+    def __init__(self, matrix: np.ndarray, axis_labels: List[str]):
         self.matrix = matrix
         self.axis_labels = axis_labels
         # Check dimensions
@@ -59,26 +60,22 @@ def __getitem__(self, key):
         )
 
     @property
-    def ndim(self):
+    def ndim(self) -> int:
         return self.matrix.ndim
 
     @property
-    def shape(self):
+    def shape(self) -> tuple:
         return self.matrix.shape
 
     @property
-    def labels(self):
-        labels = []
-        labels.extend(self.get_axis_labels(dim) for dim in range(len(self.axis_labels)))
-        return labels
+    def labels(self) -> list:
+        return [self.get_axis_labels(dim) for dim in range(len(self.axis_labels))]
 
     @property
-    def label_units(self):
-        units = []
-        units.extend(self.get_axis_labels(dim) for dim in range(len(self.axis_labels)))
-        return units
+    def label_units(self) -> list:
+        return [self.get_axis_labels(dim) for dim in range(len(self.axis_labels))]
 
-    def diag(self, k=0):
+    def diag(self, k: int = 0) -> np.ndarray:
         """
         Extract a diagonal.
 
@@ -95,7 +92,7 @@ def diag(self, k=0):
         """
         return np.diag(self.matrix, k=k)
 
-    def get_label_names(self, axis=None):
+    def get_label_names(self, axis: Optional[int] = None) -> List[List[str]]:
         """Return only the names of the labels
         along the specified axis if requested.
 
@@ -120,7 +117,7 @@ def get_label_names(self, axis=None):
         labels.extend([x[0] for x in self.get_axis_labels(dim)] for dim in r)
         return labels
 
-    def get_unique_label_names(self):
+    def get_unique_label_names(self) -> List[str]:
         """Return all unique label names (there may be duplications between axes).
 
         Returns
@@ -332,7 +329,7 @@ class DesignMatrix(PintMatrix):
     TODO: 1. add index to unit mapping.
     """
 
-    def __init__(self, matrix, labels):
+    def __init__(self, matrix: np.ndarray, labels: List[str]):
         super().__init__(matrix, labels)
 
     @property