.mother, .child{
width:50px;
height:50px;
}
.mother {
border-bottom : 50px solid red;
border-right : 50px solid red;
margin:auto;
position:relative;
right:25px;
top:25px;
}
.child {
border-top :50px solid red;
border-left: 50px solid red;
position:relative;
top:50px;
left:50px;
}