function open_voorwaarden() {
    var width  = 600;
    var height = 550;

    var left = ((screen.width - width) / 2);
    var top  = ((screen.height - height) / 2);

    window.open("general_conditions.html","Algemene_voorwaarden","scrollbars=1menubar=0,resizable=1,width=" +width+ ",height=" +height+ ",left=" + left + ",top=" + top + "");
}