f7723559_CSS_HTML_XHTML_Reference_Book__Description_and_notes
[Property/Values] border-top-width, border-right-width, border-bottom-width, border-left-width one of thin, medium, thick or a length [Description and notes] 
for defining just the border's width on only one side of an element initial value: medium; not inherited
CSS HTML XHTML Reference Book
Description and notes