The existing form to which a derivational affix can be added is called a stem. A stem can be a bound root, a free morpheme, or a derived form itself. For example, "sorrow" is the stem of the word "sorrowful". 被附加上派生词缀的那个业已存在的单词叫词干。一个词干可以是一个粘着性词根，一个自由词素，也可能本身就是一个派生形式。