diff --git a/casioplot.py b/casioplot.py
index 1be3584..0bf42fe 100644
--- a/casioplot.py
+++ b/casioplot.py
@@ -2,15 +2,15 @@
 import pygame
 
 pygame.init()
-screen_width = 384
-screen_height = 192
-screen = pygame.display.set_mode((screen_width, screen_height))
+SCREEN_WIDTH = 384
+SCREEN_HEIGHT = 192
+screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
 
 draw_string = print
 def set_pixel(x, y, color):
     pygame.draw.rect(screen, color, pygame.Rect(x, y, 1, 1))
 def get_pixel(x, y):
-    if x >= screen_width or y >= screen_height or x < 0 or y < 0:
+    if x >= SCREEN_WIDTH or y >= SCREEN_HEIGHT or x < 0 or y < 0:
         return None
     return screen.get_at((x, y))[:2]
 def clear_screen():