document.observe('dom:loaded', function() {

    $$('table.alternate_rows tr:nth-child(odd)').invoke('addClassName', 'row');
    //this adds class="row" to every odd instance of <tr> on tables that have the class name "alternate_rows"
    $$('table.alternate_rows tr:nth-child(even)').invoke('addClassName', '_row');
    //this adds class="_row" to every even instance of <tr> on tables that have the class name "alternate_rows"	
    //classes are put on both instead of just alternate rows to account for the CSS hover effect when JavaScript is turned off

});