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HTML 

              
                 <div class="container-center-all">
        <span class="title">
          <a href ="https://medium.com/free-code-camp/flexboxs-flex-basis-explained-83d1a01413b7" target="_blank">Read the medium article here</a>
        </span>
        <div class="content-section-block">
            <span class="title--secondary">
                flex-direction: <i>row;</i>
            </span>
            <span class="details">
                <b>height: 250px;</b> - <i>flex-basis: 250px;</i>
                <p>The true nature of flex-basis is best seen when using flex-wrap -
                    In this example, we are getting a block of 4 x 4 because the flex basis
                    is set at 250px in a box that is 500px <b>wide</b>, so there is only enough
                    room for 2 blocks on the <i>main axis, in this case, "Row" so horizontally</i>,
                    and the other two blocks drop below.
                    </br></br>
                    So what we see here is the flex-basis controlling the width!
                </p>
            </span>
            <span class="details"><b>Flex-basis direction &hArr; <i>horizontally</i></b></span>

            <input type="checkbox" class="btn--1-checkbox" id="btn1checkbox">
            <label for="btn1checkbox" class="btn btn--1">
                Turn <span class="btn--on-off"></span> flex-wrap
            </label>
            <div class="container-1">
                <div class="container-1__item">
                    <div class="container-1__item--inner">
                        <div class="center-numbers">1</div>
                    </div>
                </div>
                <div class="container-1__item">
                    <div class="container-1__item--inner">
                        <div class="center-numbers">2</div>
                    </div>
                </div>
                <div class="container-1__item">
                    <div class="container-1__item--inner">
                        <div class="center-numbers">3</div>
                    </div>
                </div>
                <div class="container-1__item">
                    <div class="container-1__item--inner">
                        <div class="center-numbers">4</div>
                    </div>
                </div>

                <span class="container-1__hover-direction-marker">
                    &hArr;
                </span>
                <span class="container-1__hover-details">
                    These boxes are taking on the default nature of a flex item by
                    shrinking to fit evenly due to <i>flex-shrink:</i> which is true by default.
                    The same thing would happen here if we had
                    set the width: 250px; but instead we can use flex basis when the
                    flex direction is set to row.
                </span>

                <span class="container-1__hover-details--2">
                    <i>height: 250px;</i></br>
                    <i>flex-basis: 250px;</i></br>
                    <i>flex-wrap: nowrap;</i></br>
                </span>
            </div>
        </div>

        <div class="content-section-block">
            <span class="title--secondary">
                flex-direction: <i>column;</i>
            </span>
            <span class="details">
                <b>width: 250px;</b> - <i>flex-basis: 250px;</i>
                <p>The true nature of flex-basis is best seen when using flex-wrap -
                    In this example, we are getting a block of 4 x 4 because the flex basis
                    is set at 250px in a box that is 500px <b>tall</b>, so there is only enough
                    room for 2 blocks on the <i>main axis, in this case, "Column" so vertically</i>,
                    and the other two blocks shift right.
                    </br></br>
                    So what we see here is the flex-basis controlling the height!
                </p>
            </span>
            <span class="details"><b>
                    Flex-basis direction
                    <div class="rotate-arrow">&hArr;</div>
                    <i>vertically</i></b>
            </span>
            <input type="checkbox" class="btn--2-checkbox" id="btn2checkbox">
            <label for="btn2checkbox" class="btn btn--2">
                Turn <span class="btn--on-off"></span> flex-wrap
            </label>
            <div class="container-2">
                <div class="container-2__item">
                    <div class="container-2__item--inner">
                        <div class="center-numbers">1</div>
                    </div>
                </div>
                <div class="container-2__item">
                    <div class="container-2__item--inner">
                        <div class="center-numbers">2</div>
                    </div>
                </div>
                <div class="container-2__item">
                    <div class="container-2__item--inner">
                        <div class="center-numbers">3</div>
                    </div>
                </div>
                <div class="container-2__item">
                    <div class="container-2__item--inner">
                        <div class="center-numbers">4</div>
                    </div>
                </div>

                <span class="container-2__hover-direction-marker rotate-arrow">
                    &hArr;
                </span>
                <span class="container-2__hover-details">
                    These boxes are taking on the default nature of a flex item by
                    shrinking to fit evenly due to <i>flex-shrink:</i> which is true by default.
                    The same thing would not happen here if we had
                    set the height: 250px; so as a best practice we should always use flex basis when the
                    flex direction is set to column to avoid unexpected results.
                </span>
                <span class="container-2__hover-details--2">
                    <i>width: 250px;</i></br>
                    <i>flex-basis: 250px;</i></br>
                    <i>flex-wrap: nowrap;</i></br>
                </span>
            </div>
        </div>
    </div>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Lato|Open+Sans');

$font-primary: 'Lato', sans-serif;
$font-copy-content: 'Open Sans', sans-serif;

$color-primary: #f1425d;
$color-secondary: #f17442;

$color-lightest-grey: #eee;
$color-midrange-grey: #a8a8a8;
$color-darkest-grey: #333;

$color-white: #fff;

* {
    margin: 0;
    padding: 0;
    box-sizing: inherit;
}

html {
  box-sizing: border-box;
}

body {
    background-color: $color-lightest-grey;
    font-family: $font-copy-content;

    i {
        color: $color-darkest-grey;
    }
}

::selection {
    background-color: $color-darkest-grey;
    color: $color-lightest-grey;
  }

.container-center-all {
    text-align: center;
}

.content-section-block {
    display: inline-block;
    width: 500px;

    &:not(:last-child) {
        margin-right: -5px;
    }
}

.title {
    margin-top: 20px;

    &,
    a,
    &--secondary {
        display: block;

        font-weight: 700;
        margin-bottom: 10px;
        font-size: 25px;
        font-variant: small-caps;
        letter-spacing: 1px;
        color: #000;
        font-family: $font-primary;
    }

}

.details {
    display: block;

    margin-bottom: 10px;
    font-size: 20px;
    font-variant: small-caps;
    letter-spacing: 1px;

    p {
        font-variant: normal;
        margin-top: 7px;
        padding: 0 5px 0 5px;
        font-size: 14px;
    }

}

.container-1,
.container-2 {
    position: relative;

    background-color: $color-midrange-grey;
    text-align: center;
    width: 500px;
    height: 500px;

    display: flex;
    flex-wrap: wrap;

    overflow: hidden;

    &__item {
        color: $color-white;
        font-size: 40px;
        
        // Notice that flex-basis is applied to both
        flex-basis: 250px; // Where the magic happens

        &--inner {
            position: relative;

            border: 2px solid $color-midrange-grey;

            width: 100%;
            height: 100%;
        }

    }

    &__hover-direction-marker {
        display: inline-block;

        position: absolute;

        color: $color-white;
        font-size: 80px;

        opacity: 0;
        backface-visibility: hidden;

        text-shadow: 2px 2px 4px rgba(0, 0, 0, 0.2);

    }

    &__hover-details {
        display: inline-block;

        position: absolute;

        color: $color-white;
        opacity: 0;

        &--2 {
            opacity: 0;
            transform: translateX(400px);
        }

    }

}

.container-1 {
    flex-direction: row;

    &__item {
        // flex-direction: row;
        height: 250px;

        &--inner {
            background-color: $color-primary;
        }
    }

    &__hover-direction-marker {
        top: 140px;
        left: 23px;
    }

    &__hover-details {
        bottom: 35px;
        left: 25px;
        width: 245px;
        text-align: left;

        transform: translateX(-400px);

        &--2 {
            position: absolute;

            bottom: 35px;
            right: 25px;
            text-align: right;
        }
    }
}

.container-2 {
    flex-direction: column;

    &__item {
        // flex-direction: column;
        width: 250px;

        &--inner {
            background-color: $color-secondary;
        }
    }

    &__hover-direction-marker {
        top: 3px;
        left: 160px;
    }

    &__hover-details {
        top: 20px;
        right: 25px;
        width: 200px;

        text-align: right;

        transform: translateX(400px);

        &--2 {
            position: absolute;

            bottom: 35px;
            right: 25px;
            text-align: right;
        }

    }
}

.center-numbers {
    position: relative;

    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}


.btn {
    display: inline-block;
    position: relative;

    margin-bottom: 10px;
    width: 50%;
    padding: 5px 10px;
    text-transform: uppercase;
    text-decoration: none;
    color: $color-white;
    cursor: pointer;

    box-shadow: inset -1px -1px 1px rgba(0, 0, 0, 0.4);

    &--1,
    &--2 {
        &:hover {
            opacity: .8;
        }
    }

    &--1 {
        background-color: $color-primary;
    }

    &--2 {
        background-color: $color-secondary;
    }

}

// Functionality

.btn--1-checkbox:checked~.container-1 {
    flex-wrap: nowrap;
}

.btn--2-checkbox:checked~.container-2 {
    flex-wrap: nowrap;
}

.btn--1-checkbox:checked~.container-1>.container-1__hover-direction-marker,
.btn--2-checkbox:checked~.container-2>.container-2__hover-direction-marker {
    opacity: 1;
    transition: opacity .7s .5s ease-in;
}

.btn--1-checkbox:checked~.container-1>.container-1__hover-details,
.btn--1-checkbox:checked~.container-1>.container-1__hover-details--2,
.btn--2-checkbox:checked~.container-2>.container-2__hover-details,
.btn--2-checkbox:checked~.container-2>.container-2__hover-details--2 {
    opacity: 1;
    transform: translateX(0);
    transition: all .4s;
}

.btn--1-checkbox:checked ~ .btn--1,
.btn--2-checkbox:checked ~ .btn--2  {
    opacity: .8;
    box-shadow: inset 1px 1px 2px rgba(0, 0, 0, 0.4);
    transform: translate(1px, 1px);
}

.btn--1-checkbox:checked ~ .btn--1 > .btn--on-off,
.btn--2-checkbox:checked ~ .btn--2 > .btn--on-off {
    &::before {
        display: inline-block;
    }
    &::after {
        content: "off";
        display: none;
    }
}

.btn--on-off {
    &::before {
        color: black;
        font-weight: 700;
        content: "on";
        display: none;
    }
    &::after {
        content: "off";
        display: inline-block;
    }
}

.btn--1-checkbox, .btn--2-checkbox { display: none; }
.rotate-arrow { display: inline-block; transform: rotate(90deg); }
!

JS 

              
                // please feel free to leave comments if you have any advice for changes / additions to this ;-)
!
999px

Console