A fancy Latin word for a specific type of abbreviation; the plural is sigla. Among scholars of ancient manuscripts and coins, conventional short-forms are used when referring to important sources. That way, footnotes can pack a remarkable amount of information into a very short space. To an outsider, such footnotes often seem impenetrable, but understanding them is not very difficult once you learn how to decode them.

The use of sigla is easiest to illustrate with a real example. If you are reading a copy of the New Testament with a complete apparatus criticus -- that is to say, a copy of the New Testament where every change across surviving manuscripts is footnoted -- then you might encounter a footnote that looks something like this.

1. εις τους αιωνας των αιωνων -- "forever and ever" -- א A B K L P 33 81 323 1241 Byz cav dubl hub harl tol sa^ms bo
2. εις τους αιωνας -- "forever" -- P⁷² 69 206 614 623 630 945 1505 1611 1739 2138 2495 al r am ful hark sa^mss bo^mss arm^mss

In this case the text is 1 Peter 4:11, but the content of the verse is not what I'm drawing your attention to here; rather, I want you to look at the string of letters and numbers following each phrase. Each of these is a siglum for a specific manuscript or family of manuscripts.

1. א refers to the Codex Sinaiticus, A refers to the Codex Alexandrinus, B refers to the Codex Vaticanus, and so on down the line; these are the textual witnesses that read "forever and ever" in this verse.
2. P⁷² refers to the Bodmer papyri fragments; 69 belongs to the so-called "Farrar Group" of minuscules; 206 refers to a thirteenth-century compilation of the Epistles and Acts, and so forth; these are the textual witnesses that read "forever" in this verse.

The question, then, is whether the original author of 1 Peter had written the words των αιωνων. Did a later scribe add them into a text that didn't originally include them, or did that scribe remove them from a text that originally had them? The job of the palaeographer is to trace relationships between manuscripts in order to see where and when the change crept in.

New Testament sigla follow some more or less predictable patterns. Capital letters usually refer to uncial Greek manuscripts; Arabic numerals usually refer to minuscule Greek manuscripts; the letter P with a superscripted number usually refers to a papyrus manuscript; lower-case letters usually refer to manuscripts in Old Latin. So far as I am aware, the Codex Sinaiticus is the only manuscript that is referred to by a Hebrew letter; indeed, that is not the only thing that makes Sinaiticus special.

The names of manuscript families are often both abbreviated and italicized (so, in our example above, Byz means the Byzantine family of manuscripts). Translations into regional languages such as Coptic or Syriac will usually be marked as such. In our example, arm^mss informs the scholar that multiple Armenian manuscripts attest to the second reading, though it doesn't list them all. Interestingly, there are Sahidic witnesses to both versions of this verse; an expert in that language might go further in weighing their value and establishing their provenance.

Armed with this information, then, the scholar can start the meticulous work of establishing what the original document might have said. Behind each siglum lies a manuscript that is of a certain age, that was written by a scribe with more or less skill, and that comes out of a certain geographical location; its readings must be judged against the others when an eclectic text is being compiled.

It is not important to memorize all the sigla, even if you are a professional New Testament scholar. Published lists of sigla appear in numerous reference books (and a few incomplete lists can be found online). It is probably a good idea to learn the sigla for the most important manuscripts, though, and to keep an eye out for earlier sources. Generally the papyri and the uncial codices are earlier than minuscules and translations out of Greek, but there are important exceptions.

Further Reading:

The work of Bruce Metzger is the best starting point for any study of New Testament manuscripts.
A good list of New Testament sigla can be found at Rich Elliott's excellent site for textual criticism: http://www.skypoint.com/~waltzmn/Versions.html.
Perhaps a noder more well-informed than I can provide sources for the study of other historical periods. I would also be keen to see a numismatist speak up about how sigla are used in the study of coins.

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