A grid.
3 horizontal lines.
3 vertical lines.
A horizontal line with the height highlighted.
A vertical line with the width highlighted.
A grid with the horizontal gaps highlighted.
A grid with the vertical gaps highlighted.
1
2
3
4
5
6
7
8
9

CSS

.layout {
width: 100%;
display: grid;
grid-template-rows: repeat(3, 1fr);
grid-template-columns: repeat(3, 1fr);
gap: 8px;
}

HTML

<section class="layout">
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
<div>6</div>
<div>7</div>
<div>8</div>
<div>9</div>
</section>