Why are the plays in 5 acts? asks Carrol Kindel

During a recent tour, I was asked why all Shakespeare's plays were divided into 5 acts. Not aware that they were, I said I would be happy to check into it and email back. My plan involved research, but luckily Bill Stewart was at the desk and volunteered to check his copy of Halliday's "Companion to Shakespeare" when he returned home. He did and here is my emailed explanation to the visitor, who very soon emailed back her thanks.

Greetings from the Folger,
I have found the answer to your question, Janet, from Thursday's tour at the Folger Shakespeare Library. My colleague, Bill, who was sitting at the desk when we finished our tour went home to his copy of Halliday's "Companion to Shakespeare". There he discovered that dividing plays into 5 acts was a classical convention derived from Seneca. It wasn't practiced consistently until after 1600 when Ben Jonson regularized it. The First Folio follows it in some of the plays, but not all of them with no real consistent pattern. For example, Romeo and Juliet does not follow the 5-act convention in the Folio. Plays were later divided in this way, sometimes following the French convention of using the entrances and exits of main characters as break-points. I hope this was helpful and that you and your husband enjoyed your visit to the Folger.