The Heiberg edition of the Teubner series. This is the definitive "modern" edition of all Euclid's work. It was published in parts between 1883 and 1888 in Leipzig, Germany by Teubner, who still publishes Latin and Greek editions to be used by scholars.
On page XLII, you can see a column of bold face letters followed by the abbreviation, "cod." and a name. These symbols indicate the names of the corresponding manuscripts of Euclid that have been found. For example, the one at the top, P, is the "codex Vaticanus Graecus 190, siglum X," which is Latin for "Greek Vatican book number 190, tenth century A.D." The preface, notes and abbreviations of all Teubner editions are still written in Latin, even though the books are printed in Germany.
"Manuscript" means written by hand, or rather copied by hand before the invention of the printing press. Hand copying was formerly the only way to make copies of a book. The printing press revolutionized the transmission of information in its time as much as the computer has in our own.
Every manuscript has a story to tell. This particular manuscript, P, is very important. It was discovered in Rome in 1808 by F. Peyrard, a Frenchman, in the library of the Vatican. At the time, Napoleon was collecting manuscripts from Italy and sending them back to Paris. Peyrard took advantage of the situation and had 190 sent to Paris where he studied it and came out with his own Greek edition of Euclid (Heath, The Thirteen Books of Euclid's Elements, 103). 190 turned out to be quite different from other versions and was proven to be more reliable. On page 1 you see the beginning of the actual Greek in a sort of Italics. Notice that there are no Arabic numerals. Arabic numerals are Arabic; this is Greek. The Greeks used letters to indicate sequence. Like us, they counted in base ten. This page may or may not be how Euclid's first edition looked. It's probably pretty close.
Notice the smaller print underneath the Greek with lots of numbers, letters and abbreviations. This is called the "apparatus criticus," or "critical apparatus." It tells the scholar where there are important differences between manuscripts or early editions.
The point of this piece of evidence is to show the effort that has gone into getting straight exactly what words Euclid wrote as opposed to additions, omissions and errors made later by copyists, editors and scholars.