.container {
  display: grid;
  grid-template:
    "header header" /* row 1 */
    "sidebarl main sidebarr"  /* row 2 */
    "footer footer" /* row 3 */
	/ 1fr 2fr;  /* column widths (1fr = one fraction) */
  grid-gap: 10px;   /* (optional) space between areas */
}

/* assign your elements to the areas you defined in the grid-template rule: */
header { grid-area: header; }
aside { grid-area: sidebarl; }
main { grid-area: main; }
aside { grid-area: sidebarr; }
footer { grid-area: footer; }