Since Millie Wewerka's retirement from the Library of Congress, I have been answering questions from catalogers about language codes and serving as an observer of this group.  I thought a summary of the terms used in LC cataloging for different forms of Greek might be helpful.
The term "Classical Greek" is not used in the Library of Congress Subject Headings (LCSH).  It does appear in an appendix to the Anglo-American Cataloging Rules, 2nd edition (AACR2), but I have recommended that "Ancient Greek" be substituted for it in the descriptive cataloging rules that will replace AACR2.  Strictly speaking, "Classical Greek" refers only to the language of Athens in the fifth and fourth centuries B.C.
I have recently proposed two new romanization tables: one for ancient and medieval Greek (before 1454), the other for modern Greek (after 1453).  The division is based on the date of the fall of Constantinople and the end of the Byzantine Empire, May 29, 1453.
In notes and uniform titles, we normally just use "Greek" instead of the MARC code list forms "Attic Greek," "Greek, Ancient," and "Greek, Modern."  But if the item we have is a translation from one form of the language into another (generally into modern Greek) or contains text in two different forms (one of which generally is modern Greek), we use the following terms:
Ancient Greek [before 300 B.C.]
Hellenistic Greek [300 B.C.-A.D. 600]
Biblical Greek [for the Septuagint and the New Testament; obviously, Biblical Greek is a subset of Hellenistic Greek]
Medieval Greek [600-1453]
Modern Greek [1453-present]
Why is 300 B.C. used as the beginning date for Hellenistic Greek?  After the death of Alexander in 323 B.C., several of his generals divided his empire among them and proceeded to make war against each other.  Meanwhile, Greek culture spread throughout that part of the world, and the Greek language became the lingua franca.
Why is A.D. 600 used as the beginning date for medieval Greek?  Not long thereafter, the emperor Heraclius (reigned 610-641) replaced Latin with Greek as the official language of the Eastern Roman (Byzantine) Empire, giving the empire a definitely Greek character.  The Western Roman Empire had fallen in 476.
Aside from headings for dialects (e.g., Attic Greek dialect), LCSH uses:
Greek language
Greek language, Hellenistic (300 B.C.-600 A.D.) [cross-reference from Greek language (Koinē)]
Greek language, Biblical [for works on the language of the Septuagint and the New Testament]
Greek language, Medieval and late [ca. 600-1821; note that the year of Greek independence from the Ottoman Empire is used as the end of the date range; cross-reference from Greek language, Byzantine]
Greek language, Modern [no scope note specifying dates]
If this is a request for an addition to Part 3, I recommend that the dates 600-1453 be used.  But if it is adopted as a change to Part 2, I shall have to examine many LC bibliographic records myself to determine which ones need to be changed.  Part of the problem is that many works in or translated from ancient and medieval Greek have been incorrectly coded gre instead of grc by catalogers and others who do not know that there are different MARC codes for ancient/medieval and modern Greek.
Lucas Graves ([log in to unmask])
Cooperative Cataloging Program Specialist
Data Integrity Section
Policy and Standards Division
Library of Congress
Washington, DC 20540
(Nothing in this message is to be taken as a statement of official LC policy, etc.)

>>> Joan Spanne <[log in to unmask]> 10/17/2008 10:35 AM >>>

Dear JAC,

This request to create a code element for Medieval Greek has languished far too long (my fault). I am bringing it up again for active discussion. If this request arrived in my in-box today, I would not have taken it on as simply a Part 3 change, because it really is not.

Documentation is at

The essence of the problem is that to create a Medieval Greek, we need to find room between the existing Greek code elements:

Greek, Ancient (to 1453) Greek, Ancient (to 1453) grec ancien (jusqu'à 1453) grc  
Greek, Modern (1453-) Greek, Modern (1453-) grec moderne (après 1453) gre/ell el

The overlap is with really with Ancient Greek, because of the date note.

I'd like to I suggest we could possibly handle this request by changing the scope of [grc] from individual language code element to collective code element (retaining its current name, but adding "Languages" and possibly removing the date, though the date is "instructive"), and introducing separate code elements for:
Without the date, [grc] as an individual language would be taken by most people interested in these languages to mean the same as Classical Greek, or both Classical and Hellenistic Greek. I suggest Collective rather than Macrolanguage because I am reluctant to move toward time-depth macrolanguages. I think the best answer is a true split of [grc], but I don't have thousands of instances of its use to deal with.