Early texts of Shakespeare's works
The earliest texts of William Shakespeare's works were published during the 16th and 17th centuries in quarto or folio format. Folios are large, tall volumes; quartos are smaller, roughly half the size. The publications of the latter are usually abbreviated to Q1, Q2, etc., where the letter stands for "quarto" and the number for the first, second, or third edition published.
The additional plays section in the 1664 second impression of the Third Folio.
Quarto is the format of a book or pamphlet produced from full sheets printed with eight pages of text, four to a side, then folded twice to produce four leaves. The leaves are then trimmed along the folds to produce eight book pages. Each printed page presents as one-fourth size of the full sheet.
Title page of the first quarto edition of Shakespeare's Midsummer Night's Dream, 1600, from the Folger Shakespeare Library