I'm trying to build a pattern that select the second descendant of the type"articulo" of the document: 
Do you mean an XSLT pattern or an XPath expression?
 <titulo>My title</titulo>
 <capitulo>My capitulo I</captiulo>
     <seccion>My seccion</seccion>
         <articulo><head>articulo 1</head>
             <p>My p</p>
         <artiuculo><head>articulo 2</head>

 I think that the pattern "//articulo[2]" is the correct but the XSLT
 proccesor doesn´t found it.
//articulo[2] will find every <articulo> element in the document that is the second <articulo> child of its parent element. If  you want the second <articulo> element in the document, use (//articulo)[2] - this is allowed as an XPath expression but not as an XSLT pattern, hence my earlier question.

Michael Kay
Software AG
home: Michael.H.Kay@ntlworld.com
work: Michael.Kay@softwareag.com