what to put in empty grid area css code example

Example: how to use a blank space with grid template areas css

.container {
  display: grid;

  grid-template-columns: 2fr 2fr 1fr 2fr;
  grid-template-rows: 100px 200px 100px;

  grid-template-areas:
    "head head . side"
    "main main . side"
    "footer footer footer footer";
}