Starting to use jQuery having read 'jQuery in Action'.
Came across the following sysntax for sortable tables:
  $('table.sortable').each(function() {
    var $table = $(this);
    $('th', $table).each(function(column) .....

Clearly the $('th', $table).each syntax results in traversing 'th'
elements that are descendants of each of the 'table.sortable'
elements.  I'm missing where this syntax is documented and explained.