In the book Extraordinario Libro, Sebastiano Serlio shows the reader different layouts for portals. His book is very easily structured: He identifies 20 different designs of portals, which he describes very shortly on the first pages of the book. This part can be interpreted as the table of contents. In the subsequent core of the book, he displays pictures of the previously described designs. The style of the portals varies substantially and it is clearly visible that Sebastiano Serlio does not restrict himself to the traditional roman architecture of portals. Although the motive of the pillar reappears in every portal in one form or another, the other components are interpreted more freely and vary quite a bit. The book should be understood as an appendix, which the title already implicates: It is an affix to the 7 books he wrote about architecture in which he treats the theme of portals with special care. The book presents 46 illustrations on 56 pages, which substantiates Sebastiano Serlio`s tendency to illustrate and teach by drawing rather than by describing his ideas with words. The book measures 331mm in height, 221mm in width and 19mm in depth. Therefore, the book appears to have been written in the 64mo/Sexagesimo-quarto/Sf format, as the format adheres almost perfectly to the 2 by 3 proportions.