Answer:
   
   The Tosafot are commentary on the Talmud by various Rabbis shortly
   after the time of Rashi; Many of these rabbis were descendants of
   Rashi himself.
   
   The word "Tosafot" translates as "additions" or "supplements." This
   means that their authors and editors saw their work as supplements to
   Rashi's basic commentary. Some have seen the Tosafot as an addition to
   the Talmud itself. It carries on the Talmud's own methods of
   dialectical argument and debate. The Tosafot are printed on the outer
   margin of the page; i.e., when looking at an opened book you will see
   the Tosafot in the columns closest to the edges of the pages, farthest
   from the binding. They appear in Rashi script, with the headings of
   each discussion in large square letters. The Tosafot that have been
   printed in the standard Talmud editions are merely an accidental
   selection from a vast literature that circulated in manuscript. Some
   of the other Tosafot compendia have been published as separate works.

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