When a coin is unearthed after having been buried for hundreds, or even thousands of years, it emerges covered in dirt. Oxygen in the air, acid in the ground, and salt in the soil cause copper to corrode and silver to tarnish. Although the first step of a numismatist is to clean the coin chemically, corrosion may have caused serious, irreversible damage such as pitting and scarring to the surfaces of the coins. On these poorly-preserved pieces, mint-marks, punctuation marks, smaller iconographic attributes, and legends may be difficult to discern or completely illegible. Corrosion can also cause coins to “stick” together, making it often impossible to separate individual coins without destroying them. Finally, many hoards of the 4th-5th centuries also contain blank coins which were never struck and have no identifying markings. As a result, on average 25-50% of all coins discovered at an excavation site are illegible and these are, especially in older publications, often omitted from final publications. That this is problematic for our research is evident: to compare all coin deposits, one must take into account every coin that was found, legible or not. Excluding illegible coins skews interpretations of the assemblages as a whole: small, bronze coins that are no longer legible are discarded, while gold coins, often better preserved, are kept. This creates the false impression that there was a higher proportion of gold coins in the original deposit. In this database, I have tried to note the illegible coins in every synagogue deposit as I could find them in the literature or the IAA database, but, unfortunately, many coins never made it into the database and their exact quantity and information is lost forever. A methodological consequence of this might be that we are missing the latest coin in the deposit, in which case we would establish an incorrect terminus post quem date for the assemblage, or that we are missing certain kinds of coins that would be able to tell us something on the function of the deposit (for example, when we are missing the older coins, making us interpret the deposit as an emergency hoard instead of a savings hoard, see “SYNAGOGUE COIN DEPOSITS: FUNCTIONS AND INTERPRETATION”). This will remain an obstacle that could not be overcome.
Even when coins are cleaned and legible, the numismatist may encounter other problems. For a long time, for example, coin specialists had no idea how to date a coin when no datemarks were struck. For the oldest excavations, numismatists thus mention coins found but without any attempt to date them. This problem only began to be resolved around the middle of the 20th century, when coins were discovered in more controlled excavations where they could be compared to other materials found in the same strata. Eventually, numismatists began to publish catalogues which give detailed overviews of all the coins minted by the Roman and Byzantine emperors including mints and dates. Most catalogues describe the following information about each coin, organized by emperor and mint: the type of metal, date, obverse and reverse inscription, reverse description of the image, and exergue or minting mark. Elaborate catalogues are now standard tools for any numismatist. However, catalogues are also updated as new coins are discovered and new insights learned. Thus, coins can be identified differently in various catalogues, depending on when the catalogue was published, or the personal preferences of the author.  For this project, I have provided references to the catalogues as provided by the numismatist who published the coins from that site, or by the IAA when they acquired the coins from the excavators. For my own analyses, I used the RIC and BMC catalogues. Readers should bear in mind, however, that the identifications of the coins are influenced by the catalogue used and that this could have had an influence on the analyses of the coins. Future re-evaluations of types might change some specific details, such as the dates of the coins.
 Sometimes catalogues mention the condition of a coin using abbreviations. For example, coins have been labelled as FDC (à fleur de coin, or fresh from the dies), EF (extremely fine), VF (very fine), F (fine), Fair, M (mediocre), and P (poor). Of course, these terms are not objective and great variety exists between catalogues in the determination of the grade of deterioration.
 See Burrell 2007, p. 251. Luckily, this trend has started to disappear over the last two or three decades; modern standard practice, at least in archaeology of the Mediterranean world, is now to try to clean and publish all excavated coins.
 Personal communication Donald T. Ariel. One way to work around this problem in the future would be for archaeologists to use metrology; the weighing of coins as a whole, as weight is one trait coins still retain, no matter how illegible (Grierson 1975, pp. 146–149; Burrell 2007, p. 248). Especially when lumps of coins are stuck together, weighing the lump can tell us the number of coins in the cluster. Comparing bronze hoards based on their total weight could provide relative information about the sizes of the deposits.
 For example, if a coin is found in a stratum with 5th century pottery, the coin could be assigned a 5th century terminus post quem. By combining dozens of strata, numismatists can establish relative chronologies of coin types, connecting them to specific emperors and fixed periods.
 In the future, these catalogues maybe surpassed by coin-recognition software. See for example, the work done by the Computer Vision Lab at the Vienna University of Technology (Zambanini, Kavelar, and Kampel 2013; Anwar, Anwar, Zambaninie, and Porikli 2019) or at the University of St. Andrews, in Scotland (Cooper and Arandjelovic 2019).
 New insights not only come from new archaeological discoveries but from updated methods of analysis, like testing the material used in a coin and linking it to a minting place, or taking better microscopic pictures of coins that show datable minting marks.
 We will see in our catalogue that there often are discrepancies between the published reports and the information on the same coins in the IAA database. Summary lists in excavation reports are especially occasions for mistakes: often, a numismatist will arrange the coins per locus for their own analysis, but then publish them chronologically instead. This causes errors to slip in. When a coin analysis is published in multiple reports, even more mistakes pop up. When this has happened, I have noted this in the catalogue. In most cases, the analysis of the IAA database has then been followed.
 This is of course especially true for older coin reports, which used some of the earliest coin catalogues published which now have been revised. In cases where the coins are lost (for example, at Beth Alpha), we do not have the possibility to check if the interpretations by the archaeologists are correct. Their analyses have thus been taken at face value in this project. In cases where we do have the possibility, the latest analyses by the IAA have been followed.