<h1>flex-wrap: wrap; flex-direction: row;</h1>
<ul class="flex-container longhand">
  <li class="flex-item">1</li>
  <li class="flex-item">2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4</li>
</ul>

<h1>flex-flow: row wrap;</h1>
<ul class="flex-container shorthand">
  <li class="flex-item">1</li>
  <li class="flex-item">2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4</li>
</ul>

.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
  display: flex;
}

h1 {
  padding-left: .5em; 
}

.shorthand { 
  -webkit-flex-wrap: wrap;
  flex-wrap: wrap;
  -webkit-flex-direction: row;
  flex-direction: row;
}

.longhand { 
  -webkit-flex-flow: wrap row;
  flex-flow: wrap row;
  
}

.longhand li {
  background: deepskyblue;
}

.flex-item {
  background: tomato;
  padding: 5px;
  width: 100px;
  height: 100px;
  margin: 10px;
  
  line-height: 100px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}