.matrix {
    position: relative;
    margin: 0 auto;
}

.matrix::before, .matrix::after {
    content: "";
    position: absolute;
    top: 0;
    border: 1px solid #000;
    width: 6px;
    height: 100%;
}

.matrix::before {
    left: -6px;
    border-right: 0px;
}

.matrix::after {
    right: -6px;
    border-left: 0px;
}

.matrix td {
    padding: 5px;
    text-align: center;
}