A block is simply a batch of notes whose serial numbers share the same prefix and suffix letters. So, for example, the C..A block would include serials C00000001A, C12345678A, and so forth. In recent series, for most denominations, a complete block of serial numbers runs from 00000001 to 96000000; but sometimes partial blocks are printed. The number in the "Blocks" column of the Series Statistics table is the total number of different blocks, complete or partial, printed for a given district.
If a single block is printed in several different variations, these are referred to as complex blocks. For example, if part of the C..A block was printed at Washington, DC, and part of the same block was printed at Fort Worth, then these would be considered different complex blocks. Likewise, the count of complex blocks distinguishes web notes from non-web notes in the same block, and COPE from conventional or from LEPE notes in the same block. More often than not, though, there is only one complex block for each simple block.
Breaking down the serial ranges even more finely, we reach the level of groups. Here, if production within a single block switches back and forth between varieties several times, each switch is considered to begin a new group. For example, consider the F..N block of the Series 1988A $1's. This block includes some notes printed at Washington and some at Fort Worth; and in addition, some of the notes printed at Washington are web notes. So there are three complex blocks (FNdc, FNweb, and FNfw); but because of the sequence in which the various notes were printed, there are four groups:
An exception to the rule is that, if two different sorts of notes are mixed in a single print run (as with web and non-web notes in 1988A B..L run 6), producing a serialling pattern that alternates back and forth between the two varieties once per plate position, then that run is counted as two overlapping groups, rather than 64 tiny groups. This inconsistency in the counting of groups exists mostly for practical reasons: Tracking down notes from all of these tiny serial ranges, often each only a few thousand serials long, would be far more tedious than most group collectors could stand.
This category is fairly self-explanatory. In most cases the number of print runs is simply the number of notes printed divided by the length of the standard print run. Occasionally, some oddities occur when the BEP changes its standard print run in the middle of a series. In the star notes, however, it is not unusual for some print runs to be smaller than the standard length.
The only quirk here is the way in which groups are counted in the star notes. Since the BEP has changed its numbering conventions for partial star runs several times over the years, and since all of the various numbering conventions have tended to lead to many gaps in the ranges of star serials printed, it is in general not practical to define "group" for star notes in the same way as for regular notes. For purposes of these statistics, groups in the star notes are generally equivalent to print runs, with two exceptions. First, in a few cases between 1995 and 1999, the BEP reported that a single run was printed as two sub-runs; in these cases the two sub-runs are counted as separate groups, since they are commonly collected separately. Second, before Series 1974 (1977 for $50's and $100's), consecutive star runs without internal gaps are merged into a single group as would be done in the regular notes, because the number of star runs printed in this era was great enough that a collection by print runs would be excessively large.
Thus in the star notes, some groups may contain internal gaps, and from Series 1974 onward some groups may not be separated by gaps from the preceding or following groups. This usage of the term "group" is admittedly inconsistent, but it does reflect the way many collectors actually define a complete set for a given series.
This category consists mostly of the uncut sheets that the BEP sells to collectors, but occasionally some other BEP premium products must be included. For the blocks and the complex blocks, two different counts are given: The first count includes only the special-printing (complex) blocks which are different from the circulation-printing (complex) blocks, while the second count includes all the special-printing (complex) blocks. For the groups, only one count is necessary, since the special-printing groups never duplicate the circulation-printing groups. (In particular, BEP premium products consisting of notes from the ordinary circulation print runs do not figure into these counts.)