If one filter mechanism carries chromatic filters and another carries ``neutral'' filters, the total number of bands (i.e., rows) in the table should be the product of the number of positions in the two carriers. Thus, a 4-position main filter wheel and a 3-position attenuator in the same measuring beam give 12 total passband combinations. These may be described as a single logical filter mechanism if the positions of the two wheels are indicated in adjacent data columns that can be combined into a single code field. They may equally well be treated as two logical filter carriers with separate code columns.
It is also possible to have two filter wheels in series, arranged so that one wheel carries chromatic filters for one photometric system, but ``neutral'' filters for another, whose chromatic filters are in the second wheel. Then only the combinations that make sense need be included in the instrument table; those that would put two neutral filters in series, or two chromatic filters of different systems, can be omitted. (Note that there might be useful combinations of two chromatic filters: if one wheel contains UBV filters and the other contains uvby filters, one might combine a V filter with u to give a uRL combination.)