Within the whorl group the subdivision type accidental is used for
extension purposes only. In general classification it is designated by
the letter W and for extension purposes by the letter X.
The accidental whorl is a pattern consisting of a combination of two
different types of pattern, with the exception of the plain arch, with
two or more deltas; or a pattern which possesses some of the
requirements for two or more different types; or a pattern which
conforms to none of the definitions. It may be a combination of loop
and tented arch, loop and whorl, loop and central pocket loop, double
loop and central pocket loop, or other such combinations. The plain
arch is excluded as it is rather the absence of pattern than a
pattern. Underneath every pattern there are ridges running from one
side to the other, so that if it were not excluded every pattern but
the plain arch would be an accidental whorl.
This subclassification also includes those exceedingly unusual
patterns which may not be placed by definition into any other classes.
Figures 269 to 271 are accidentals combining a loop with a tented
arch. Figures 272 to 276 combine a loop and a plain whorl or central
pocket loop. Figure 277 combines a loop and a double loop. Figure 278
combines a loop and a plain arch, so it is classified as a loop.
Figure 279 combines a loop and a tented arch.
Some whorls may be found which contain ridges conforming to more than
one of the whorl subdivisions described. In such cases, the order of
preference (if any practical distinction need be made) should be: (1)
accidental, (2) double loop, (3) central pocket loop, (4) plain whorl.