diff --git a/cudoku.c b/cudoku.c
index 19c9772..9d63c97 100644
--- a/cudoku.c
+++ b/cudoku.c
@@ -1,4 +1,5 @@
 #include <glad/gl.h>
+#include <stdio.h>
 
 #include "shader.h"
 
@@ -49,5 +50,22 @@ void draw_bg_rect(Shader shader, unsigned int vao, float x_scale, float y_scale)
   glBindVertexArray(vao);
   glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
 
-  glBindVertexArray(0);
+  // TODO: no idea why binding to default (0) breaks the grid shader
+  /* glBindVertexArray(0); */
+}
+
+void draw_bg_lines(Shader shader, float x_scale, float y_scale, float resolution) {
+  use_shader(shader);
+
+  float mat[4][4] = {
+    {x_scale, 0, 0, 0},
+    {0, y_scale, 0, 0},
+    {0, 0, 1, 0},
+    {0, 0, 0, 1},
+  };
+
+  set_mat4(shader, "transform", (float *)mat);
+  set_float(shader, "resolution", resolution);
+
+  glDrawArrays(GL_TRIANGLES, 0, 6);
 }
diff --git a/cudoku.h b/cudoku.h
index 27a475c..61eacd0 100644
--- a/cudoku.h
+++ b/cudoku.h
@@ -4,3 +4,4 @@
 
 unsigned int prepare_bg_rect(Shader shader);
 void draw_bg_rect(Shader shader, unsigned int vao, float x_scale, float y_scale);
+void draw_bg_lines(Shader shader, float x_scale, float y_scale, float resolution);
diff --git a/main.c b/main.c
index 4643cef..eb6bef1 100644
--- a/main.c
+++ b/main.c
@@ -1,7 +1,6 @@
 #include <stdbool.h>
 
 #include <glad/glx.h>
-#include <stdio.h>
 
 #include "x11.h"
 #include "cudoku.h"
@@ -39,6 +38,7 @@ int main() {
   float x_scale = 1.f, y_scale = 1.f;
 
   Shader board_shader = create_shader("shaders/board_v.glsl", "shaders/board_f.glsl");
+  Shader grid_shader = create_shader("shaders/grid_v.glsl", "shaders/grid_f.glsl");
   unsigned int board_vao = prepare_bg_rect(board_shader);
 
   bool quit = false;
@@ -69,6 +69,7 @@ int main() {
     glClear(GL_COLOR_BUFFER_BIT);
 
     draw_bg_rect(board_shader, board_vao, x_scale, y_scale);
+    draw_bg_lines(grid_shader, x_scale, y_scale, width < height ? width : height);
 
     glXSwapBuffers(display, window);
   }
diff --git a/shader.c b/shader.c
index cfeb986..3946250 100644
--- a/shader.c
+++ b/shader.c
@@ -86,3 +86,7 @@ void set_mat4(Shader shader, const char *name, float *mat4) {
   glUniformMatrix4fv(loc, 1, GL_FALSE, mat4);
 }
 
+void set_float(Shader shader, const char *name, float val) {
+  int loc = glGetUniformLocation(shader.program, name);
+  glUniform1f(loc, val);
+}
diff --git a/shader.h b/shader.h
index d237775..1a5d9ca 100644
--- a/shader.h
+++ b/shader.h
@@ -8,3 +8,4 @@ void read_shader_file(const char *path, char **buf);
 Shader create_shader(const char *vertex_path, const char *fragment_path);
 void use_shader(Shader shader);
 void set_mat4(Shader shader, const char *name, float *mat);
+void set_float(Shader shader, const char *name, float val);
diff --git a/shaders/grid_f.glsl b/shaders/grid_f.glsl
new file mode 100644
index 0000000..017ee03
--- /dev/null
+++ b/shaders/grid_f.glsl
@@ -0,0 +1,47 @@
+#version 330 core
+
+out vec4 FragColor;
+
+uniform mat4 transform;
+uniform float resolution;
+
+void main()
+{
+    // Define the size of the grid
+    int gridSize = 9;
+    
+    // Calculate the step size for the grid lines
+    float stepSize = 2.0 / float(gridSize);
+
+    // Calculate the UV coordinates of the current fragment
+    vec2 uv = (gl_FragCoord * transform).xy / vec2(resolution, resolution); // Adjust the resolution as needed
+
+    // Calculate the coordinates of the current fragment in the [-1, 1] range
+    float fragCoordX = (uv.x - 0.5) * (2.0 / transform[0][0]) - 1.0;
+    float fragCoordY = (uv.y - 0.5) * (2.0 / transform[1][1]) - 1.0;
+    
+    // Initialize colors for grid lines and background
+    vec3 gridColor = vec3(0.4, 0.4, 0.4);
+    vec3 gridColorThick = vec3(0.2, 0.2, 0.2);
+
+    bool isInsideGridHorizontally = (fragCoordX >= -2.0 && fragCoordX <= 0.0);
+    bool isInsideGridVertically = (fragCoordY >= -2.0 && fragCoordY <= 0.0);
+    bool isInsideGrid = isInsideGridHorizontally && isInsideGridVertically;
+
+    // Determine if the current fragment is on a thicker grid line
+    bool isThickLineX = isInsideGrid && mod(fragCoordX, stepSize * 3) < 0.01;
+    bool isThickLineY = isInsideGrid && mod(fragCoordY, stepSize * 3) < 0.01;
+
+    // Check if the current fragment is on a grid line
+    bool isGridX = isInsideGrid && mod(fragCoordX, stepSize) < 0.005;
+    bool isGridY = isInsideGrid && mod(fragCoordY, stepSize) < 0.005;
+    
+    // Set the fragment color based on whether it's a grid line or a thicker grid line
+    if ((isThickLineX) || (isThickLineY)) {
+        FragColor = vec4(gridColorThick, 1.0);
+    } else if (isGridX || isGridY) {
+        FragColor = vec4(gridColor, 1.0);
+    } else {
+        discard;
+    }
+}
diff --git a/shaders/grid_v.glsl b/shaders/grid_v.glsl
new file mode 100644
index 0000000..6b842d7
--- /dev/null
+++ b/shaders/grid_v.glsl
@@ -0,0 +1,12 @@
+#version 330 core
+uniform mat4 transform;
+
+vec3 gridPlane[6] = vec3[](
+    vec3(1, 1, 0), vec3(-1, -1, 0), vec3(-1, 1, 0),
+    vec3(-1, -1, 0), vec3(1, 1, 0), vec3(1, -1, 0)
+);
+
+void main() {
+  vec3 p = gridPlane[gl_VertexID].xyz;
+  gl_Position = vec4(p, 1.0f);
+}
\ No newline at end of file