diff --git a/board.py b/board.py
index 677a5dd..cb18153 100644
--- a/board.py
+++ b/board.py
@@ -30,14 +30,39 @@ class Board:
         self.__assert_lines_and_columns_have_same_values()
 
     def __str__(self):
-        return "\n".join(self.lines)
+        output = ""
+        for line in self.lines:
+            for item in line:
+                if isinstance(item, set):
+                    if len(item) == 1:
+                        output += str(item)[1:-1]
+                    else:
+                        output += '.'
+                elif isinstance(item, Square):
+                    if len(item.content) == 1:
+                        output += str(item.content)[1:-1]
+                    else:
+                        output += '.'
+                else:
+                    output += item
+                output += ' '
+            output += '\n'
+
+        return output
 
     def __assert_lines_and_columns_have_same_values(self):
-        differences = [
-            'X' for col, column in enumerate(self.columns)
-            for lin, line, in enumerate(self.lines)
-            if self.lines[lin][col] != self.columns[col][lin]
-        ]
+        differences = []
+        for lin in range(len(self.lines)):
+            for col in range(len(self.columns)):
+                if isinstance(self.lines[lin][col], Square):
+                    line_content = self.lines[lin][col].content
+                    column_content = self.columns[col][lin].content
+                else:
+                    line_content = self.lines[lin][col]
+                    column_content = self.columns[col][lin]
+
+                if line_content != column_content:
+                    differences.append('X')
 
         assert len(differences) == 0
 
@@ -58,7 +83,7 @@ class Board:
                         if str(number) not in line
                     }
                 else:
-                    self.lines[line_index][column] = {line[column]}
+                    self.lines[line_index][column] = {int(line[column])}
 
     def __compute_possible_values_from_columns(self):
         """Compute possible values for each number in a column.
@@ -77,7 +102,7 @@ class Board:
                         if str(number) not in column
                     }
                 else:
-                    self.columns[column_index][line] = {column[line]}
+                    self.columns[column_index][line] = {int(column[line])}
 
     def compute_possible_values(self):
         """Compute possible values for each square in the board."""
@@ -111,10 +136,4 @@ class Board:
                 self.lines[lin][col] = Square(lin, col, line[col])
                 self.columns[col][lin] = Square(lin, col, column[lin])
 
-        differences = [
-            'X' for col, column in enumerate(self.columns)
-            for lin, line, in enumerate(self.lines)
-            if self.lines[lin][col].content != self.columns[col][lin].content
-        ]
-
-        assert len(differences) == 0
+        self.__assert_lines_and_columns_have_same_values()
diff --git a/main.py b/main.py
index 6b302cf..22dec71 100644
--- a/main.py
+++ b/main.py
@@ -26,6 +26,8 @@ def main():
     board.intersect_possible_values()
     board.create_squares()
 
+    print(board)
+
 
 if __name__ == '__main__':
     main()
diff --git a/square.py b/square.py
index 01269eb..b7e1ebd 100644
--- a/square.py
+++ b/square.py
@@ -24,3 +24,26 @@ class Square:
             3 4 5
             6 7 8
         """
+
+        grids = [
+            (0, 0),  # grid 0
+            (0, 1),  # grid 1
+            (0, 2),  # grid 2
+            (1, 0),  # grid 3
+            (1, 1),  # grid 4
+            (1, 2),  # grid 5
+            (2, 0),  # grid 6
+            (2, 1),  # grid 7
+            (2, 2),  # grid 8
+        ]
+
+        grid_coordinates = (self.line // 3, self.column // 3)
+
+        grid_number = 0
+        while grids[grid_number] != grid_coordinates:
+            grid_number += 1
+
+            if grid_number == 9:
+                return None
+
+        return grid_number
diff --git a/tests.py b/tests.py
new file mode 100644
index 0000000..2198800
--- /dev/null
+++ b/tests.py
@@ -0,0 +1,40 @@
+"""All tests for the Sudoku Solver."""
+
+import unittest
+from square import Square
+
+
+class TestSquare(unittest.TestCase):
+    """TestCase for Square"""
+
+    def setUp(self):
+        content = '.'
+        self.board = [
+            Square(line, column, content)
+            for line in range(9)
+            for column in range(9)
+        ]
+
+        self.assertEqual(len(self.board), 81)
+
+    def test_grid_is_correctly_computed(self):
+        """Square.grid is correctly computed for all values."""
+
+        correct_grids = (
+            "000111222"
+            "000111222"
+            "000111222"
+            "333444555"
+            "333444555"
+            "333444555"
+            "666777888"
+            "666777888"
+            "666777888"
+        )
+
+        for i, square in enumerate(self.board):
+            self.assertEqual(square.grid, int(correct_grids[i]))
+
+
+if __name__ == '__main__':
+    unittest.main()