displays_vertices.c

    const float vertexPositions[] = {
        a, 0.0f, -4.0f, 1.0f,
        -a, 0.0f, -4.0f, 1.0f,
        0.0f, b, -4.0f, 1.0f,
        0.0f, -b, -4.0f, 1.0f
    };

    GLuint gridVBO;
    glGenBuffers(1, &gridVBO);

    glBindBuffer(GL_ARRAY_BUFFER, gridVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexPositions), vertexPositions, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

does_not_display_vertices.c

    const float vertexPositions[] = {
        a, 0.0f, -4.0f, 1.0f,
        -a, 0.0f, -4.0f, 1.0f,
        0.0f, b, -4.0f, 1.0f,
        0.0f, -b, -4.0f, 1.0f
    };

    const int indices[] = {
        0, 1, 2, 3
    };

    GLuint gridVAO;
    glGenVertexArrays(1, &gridVAO);
    glEnableVertexAttribArray(gridVAO);

    GLuint gridVBO;
    glGenBuffers(1, &gridVBO);

    glBindBuffer(GL_ARRAY_BUFFER, gridVBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertexPositions), vertexPositions, GL_STATIC_DRAW);

    // We also need an IBO
    GLuint gridIBO;
    glGenBuffers(1, &gridIBO);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, gridIBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(gridVAO, 4, GL_FLOAT, GL_FALSE, 0, 0);