William F. Hammond The GELLMU article document type has an endofsentence mark eos, which is a definedempty XML element, corresponding to the concept of sentence in languages such as English and French But there is no provision for regarding a western sentence itself as an XML element Why There are two reasons Sometimes one wants to begin a display in the middle of a sentence Then it can happen that the display is the last part of the sentence It is a formal rule of XML that if an element begins inside another element, then the second element must be closed before the first element is closed Following this rule, when a display is the last part of a sentence, the display must be ended before the sentence is ended As a consequence an XML processor must usually work very hard to place the sentenceending punctuation mark correctly Is this just a technical XML issue Not really The second reason for modeling an endofsentence mark but not a sentence is that some literary use of a language such as English does not actually resolve into clean sentence units even though endofsentence punctuation is used One could argue that when a sentence is used, it could be marked up with a sentence elementThe model would then likely permit each of sentence and display to contain the other In that event it is unlikely that authors would want to be required to insert endofsentence marks explicitly Moreover, there would be something of a dilemma for the XML processor if it happens to notice an item of CDATA at the end of a sentence that appears to be an endofsentence mark There would still be the vexation caused by a display that ends a sentence And would authors use the sentence element Will authors want to use the explicit eos rather than the simple CDATA punctuation mark . If so, how is the sequence .eos to be handled by a processor Authors are the end users, and authors need convenience Reasonable convenience lies in the convention that began with the dawn of the mechanical typewriter: A sentence is ended with a period followed either by a newline or by two or more blank spaces Handling this convention is not a reasonably efficient task for an XML processor But it works very well with a like markup interface for XML, i.e., when there is preprocessing from like markup to XML markup