diff --git a/src/snake_game/__init__.py b/src/snake_game/__init__.py
new file mode 100644
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--- /dev/null
+++ b/src/snake_game/__init__.py
@@ -0,0 +1,13 @@
+## @package snake_game
+#  @brief Snake game from Udemy's 100 Days of Code (day 21).
+#
+#  @mainpage Snake Game
+#
+#  A snake of three square segments moves across a Python turtle screen by
+#  itself and is steered with the arrow keys. It eats food, grows and keeps a
+#  score, and the game is over when the head passes the wall or touches the
+#  tail. The game uses the standard library only.
+#
+#  The constants of the game are in constants.py.
+
+"""Snake game from Udemy's 100 Days of Code (day 21)."""
diff --git a/src/snake_game/constants.py b/src/snake_game/constants.py
new file mode 100644
index 0000000..4feb855
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+++ b/src/snake_game/constants.py
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+## @file constants.py
+#  @brief Constants of the snake game.
+#
+#  Every value that the day-20 and day-21 lectures fix lives here, so that no other
+#  module contains a magic number. The turtle coordinate system has its centre
+#  at (0, 0); the 600 by 600 screen therefore reaches from -300 to 300 on both
+#  axes. A direction is a turtle heading in degrees: 0 points right and the
+#  angle grows counter-clockwise.
+
+"""Constants of the snake game, as fixed by the day-20 and day-21 lectures."""
+
+from typing import Final
+
+## @brief Width of the screen in pixels.
+SCREEN_WIDTH: Final[int] = 600
+
+## @brief Height of the screen in pixels.
+SCREEN_HEIGHT: Final[int] = 600
+
+## @brief Background colour of the screen.
+SCREEN_BACKGROUND_COLOR: Final[str] = "black"
+
+## @brief Title of the game window.
+SCREEN_TITLE: Final[str] = "My Snake Game"
+
+## @brief Shape of every segment of the snake.
+SEGMENT_SHAPE: Final[str] = "square"
+
+## @brief Colour of every segment of the snake.
+SEGMENT_COLOR: Final[str] = "white"
+
+## @brief Where the segments are drawn when the game starts, from head to tail.
+#
+#  The positions lie on one row, 20 pixels apart, which is the width of a
+#  default turtle square, so the segments touch without overlapping.
+STARTING_POSITIONS: Final[tuple[tuple[int, int], ...]] = ((0, 0), (-20, 0), (-40, 0))
+
+## @brief Distance in pixels that the head moves in one move.
+MOVE_DISTANCE: Final[int] = 20
+
+## @brief Direction up, as a turtle heading in degrees.
+UP: Final[int] = 90
+
+## @brief Direction down, as a turtle heading in degrees.
+DOWN: Final[int] = 270
+
+## @brief Direction left, as a turtle heading in degrees.
+LEFT: Final[int] = 180
+
+## @brief Direction right, as a turtle heading in degrees.
+RIGHT: Final[int] = 0
+
+## @brief Wait in seconds between two moves.
+REFRESH_DELAY_SECONDS: Final[float] = 0.1
+
+## @brief Shape of the food.
+FOOD_SHAPE: Final[str] = "circle"
+
+## @brief Stretch factor of the food in both directions; 0.5 is half a default turtle.
+FOOD_SIZE: Final[float] = 0.5
+
+## @brief Colour of the food.
+FOOD_COLOR: Final[str] = "blue"
+
+## @brief Speed of the food; `fastest` switches its animation off.
+FOOD_SPEED: Final[str] = "fastest"
+
+## @brief How far from the centre the wall is, on every side, in pixels.
+#
+#  The head must stay inside it, and the food is shown inside it.
+WALL_LIMIT: Final[int] = 280
+
+## @brief The snake eats the food when the head is closer to it than this, in pixels.
+FOOD_COLLISION_DISTANCE: Final[int] = 15
+
+## @brief Text in front of the score on the scoreboard.
+SCORE_LABEL: Final[str] = "Score: "
+
+## @brief Colour of the scoreboard text.
+SCOREBOARD_COLOR: Final[str] = "white"
+
+## @brief Where the scoreboard text is written: the top centre of the screen.
+SCOREBOARD_POSITION: Final[tuple[int, int]] = (0, 270)
+
+## @brief Alignment of the scoreboard text around its position.
+SCOREBOARD_ALIGNMENT: Final[str] = "center"
+
+## @brief Font of the scoreboard text: family, size and style.
+SCOREBOARD_FONT: Final[tuple[str, int, str]] = ("Arial", 24, "normal")
+
+## @brief The head touches the tail when it is closer to a segment than this, in pixels.
+TAIL_COLLISION_DISTANCE: Final[int] = 10
+
+## @brief Text shown when the game is over.
+GAME_OVER_TEXT: Final[str] = "GAME OVER"
+
+## @brief Where the game-over text is written: the centre of the screen.
+GAME_OVER_POSITION: Final[tuple[int, int]] = (0, 0)
diff --git a/tests/test_constants.py b/tests/test_constants.py
new file mode 100644
index 0000000..89a3bc2
--- /dev/null
+++ b/tests/test_constants.py
@@ -0,0 +1,127 @@
+"""Tests of the constants that the day-20 and day-21 lectures fix."""
+
+from snake_game import constants
+
+HALF_WIDTH = constants.SCREEN_WIDTH / 2
+HALF_HEIGHT = constants.SCREEN_HEIGHT / 2
+
+
+def test_screen_size_matches_lecture() -> None:
+    assert (constants.SCREEN_WIDTH, constants.SCREEN_HEIGHT) == (600, 600)
+
+
+def test_screen_is_black_and_titled_as_in_the_lecture() -> None:
+    assert constants.SCREEN_BACKGROUND_COLOR == "black"
+    assert constants.SCREEN_TITLE == "My Snake Game"
+
+
+def test_segments_are_white_squares() -> None:
+    assert constants.SEGMENT_SHAPE == "square"
+    assert constants.SEGMENT_COLOR == "white"
+
+
+def test_snake_starts_with_three_segments_at_the_lecture_positions() -> None:
+    assert constants.STARTING_POSITIONS == ((0, 0), (-20, 0), (-40, 0))
+
+
+def test_starting_positions_are_distinct_on_one_row() -> None:
+    positions = constants.STARTING_POSITIONS
+
+    assert len(set(positions)) == len(positions)
+    assert len({y for _, y in positions}) == 1
+
+
+def test_starting_positions_run_from_head_to_tail_leftwards() -> None:
+    x_values = [x for x, _ in constants.STARTING_POSITIONS]
+
+    assert x_values == sorted(x_values, reverse=True)
+
+
+def test_starting_positions_are_one_move_distance_apart() -> None:
+    x_values = [x for x, _ in constants.STARTING_POSITIONS]
+    gaps = {left - right for left, right in zip(x_values, x_values[1:], strict=False)}
+
+    assert gaps == {constants.MOVE_DISTANCE}
+
+
+def test_starting_positions_are_inside_the_screen() -> None:
+    assert all(
+        abs(x) < HALF_WIDTH and abs(y) < HALF_HEIGHT
+        for x, y in constants.STARTING_POSITIONS
+    )
+
+
+def test_move_distance_is_twenty_pixels() -> None:
+    assert constants.MOVE_DISTANCE == 20
+
+
+def test_directions_match_the_lecture_headings() -> None:
+    assert constants.UP == 90
+    assert constants.DOWN == 270
+    assert constants.LEFT == 180
+    assert constants.RIGHT == 0
+
+
+def test_four_directions_are_distinct() -> None:
+    directions = (constants.UP, constants.DOWN, constants.LEFT, constants.RIGHT)
+
+    assert len(set(directions)) == len(directions)
+
+
+def test_opposite_directions_differ_by_half_a_turn() -> None:
+    assert (constants.UP - constants.DOWN) % 360 == 180
+    assert (constants.LEFT - constants.RIGHT) % 360 == 180
+
+
+def test_refresh_delay_is_a_tenth_of_a_second() -> None:
+    assert constants.REFRESH_DELAY_SECONDS == 0.1
+
+
+def test_food_is_a_small_blue_circle_without_animation() -> None:
+    assert constants.FOOD_SHAPE == "circle"
+    assert constants.FOOD_SIZE == 0.5
+    assert constants.FOOD_COLOR == "blue"
+    assert constants.FOOD_SPEED == "fastest"
+
+
+def test_wall_is_inside_the_screen() -> None:
+    assert 0 < constants.WALL_LIMIT < HALF_WIDTH
+    assert constants.WALL_LIMIT < HALF_HEIGHT
+
+
+def test_wall_is_280_pixels_from_the_centre() -> None:
+    assert constants.WALL_LIMIT == 280
+
+
+def test_food_is_eaten_closer_than_15_pixels_which_is_less_than_a_move() -> None:
+    assert constants.FOOD_COLLISION_DISTANCE == 15
+    assert constants.FOOD_COLLISION_DISTANCE < constants.MOVE_DISTANCE
+
+
+def test_scoreboard_is_white_text_at_the_top_centre_inside_the_screen() -> None:
+    x, y = constants.SCOREBOARD_POSITION
+
+    assert constants.SCOREBOARD_COLOR == "white"
+    assert constants.SCOREBOARD_ALIGNMENT == "center"
+    assert (x, y) == (0, 270)
+    assert abs(x) < HALF_WIDTH
+    assert 0 < y < HALF_HEIGHT
+
+
+def test_scoreboard_text_is_score_label_and_arial_24() -> None:
+    assert constants.SCORE_LABEL == "Score: "
+    assert constants.SCOREBOARD_FONT == ("Arial", 24, "normal")
+
+
+def test_tail_is_touched_closer_than_10_pixels_which_is_less_than_a_move() -> None:
+    assert constants.TAIL_COLLISION_DISTANCE == 10
+    assert 0 < constants.TAIL_COLLISION_DISTANCE < constants.MOVE_DISTANCE
+
+
+def test_game_over_text_is_written_at_the_centre_inside_the_wall() -> None:
+    x, y = constants.GAME_OVER_POSITION
+
+    assert constants.GAME_OVER_TEXT == "GAME OVER"
+    assert (x, y) == (0, 0)
+    assert abs(x) < constants.WALL_LIMIT
+    assert abs(y) < constants.WALL_LIMIT