CSS Border Radius Second Radius Explanation

Question

I was reading the docs at https://developer.mozilla.org/en/docs/Web/CSS/border-radius

The border radius attribute seems to have a second radius value,

/* The syntax of the second radius allows one to four values */
/* (first radius values) / radius */
border-radius: 10px 5% / 20px;

/* (first radius values) / top-left-and-bottom-right | top-right-and-bottom-left */
border-radius: 10px 5% / 20px 30px;

/* (first radius values) / top-left | top-right-and-bottom-left | bottom-right */
border-radius: 10px 5px 2em / 20px 25px 30%;

/* (first radius values) / top-left | top-right | bottom-right | bottom-left */
border-radius: 10px 5% / 20px 25em 30px 35em;

I am aware of the border radius attribute and it's short syntax but I don't understand what does the values after '/' do.

Answer 1

The first set of values defines the horizontal radius. The optional second set of values, preceded by a ‘/’ , defines the vertical radius. If only one set of values is supplied, it is used for both the vertical and horizontal radius.