<COLGROUP> describes a group of columns in the table and let you to set the properties of those columns. <COLGROUP> goes after the <TABLE> tag immediately and before any <TR>. <COLGROUP> works very much similar to <COL>, but you must note that <COLGROUP> need both an opening and a closing tag.
<TABLE BORDER=1 CELLPADDING=4 RULES=GROUPS FRAME=BOX>
<COLGROUP></COLGROUP>
<COLGROUP SPAN=3></COLGROUP>