flex-wrap

flex-wrap: nowrap; /* 既定値 */
flex-wrap: wrap;
flex-wrap: wrap-reverse;

/* グローバル値 */
flex-wrap: inherit;
flex-wrap: initial;
flex-wrap: unset;

nowrap | wrap | wrap-reverse

<h4>This is an example for flex-wrap:wrap </h4>
<div class="content">
  <div class="red">1</div>
  <div class="green">2</div>
  <div class="blue">3</div>
</div>
<h4>This is an example for flex-wrap:nowrap </h4>
<div class="content1">
  <div class="red">1</div>
  <div class="green">2</div>
  <div class="blue">3</div>
</div>
<h4>This is an example for flex-wrap:wrap-reverse </h4>
<div class="content2">
  <div class="red">1</div>
  <div class="green">2</div>
  <div class="blue">3</div>
</div>

CSS

/* Common Styles */
.content,
.content1,
.content2 {
    color: #fff;
    font: 100 24px/100px sans-serif;
    height: 150px;
    text-align: center;
}

.content div,
.content1 div,
.content2 div {
    height: 50%;
    width: 50%;
}
.red {
    background: orangered;
}
.green {
    background: yellowgreen;
}
.blue {
    background: steelblue;
}

/* Flexbox Styles */
.content {
    display: flex;
    flex-wrap: wrap;
}
.content1 {
    display: flex;
    flex-wrap: nowrap;
}
.content2 {
    display: flex;
    flex-wrap: wrap-reverse;
}