CSS Property:
flex-flow:

Note: Internet Explorer and Safari do not support the flex-direction property.

Result:
A
B
C
D
E
F
CSS Code:
#main
{
width:150px;
height: 150px;
border:1px solid black;
display: flex;
flex-flow: row nowrap;
}
Click the property values above to see the result
W3Schools.com - Play it