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CSS Property:
flex-flow:
row nowrap
row-reverse nowrap
column nowrap
column-reverse nowrap
row wrap
row-reverse wrap
column wrap
column-reverse wrap
row wrap-reverse
row-reverse wrap-reverse
column wrap-reverse
column-reverse wrap-reverse
initial
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
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