In such cases, the beginning and end of every element must be explicitly marked, because there are no identifiable rules about which elements can appear where. In practice, however, rules can usually be formulated to reduce the need for so much tagging. A poem always has a single title element which precedes the first stanza and contains no other elements. Apart from the title, a poem consists only of stanzas. Stanzas consist only of lines and every line is contained by a stanza.
Nothing can follow a stanza except another stanza or the end of a poem.
Nothing can follow a line except another line or the start of a new stanza. From these rules, it may be inferred that we do not need to mark the ends of stanzas or lines explicitly. O Rose thou art sick. Does thy life destroy. Before considering these rules further, you may wish to consider how text marked up in the form above could be processed by a computer for very many different purposes.