As devised by McIntosh, a 3-letter designation of the white-light characteristics of a sunspot group. The general form of the designation is Zpc. One letter is chosen from each of the following three categories.
Z (the modified Zurich class of the group):
A-A small single sunspot or very small group of spots with the same magnetic polarity, without penumbra.
B-Bipolar sunspot group with no penumbra.
C-An elongated bipolar sunspot group. One sunspot must have penumbra, and penumbra does not exceed 5 ° in longitudinal extent.
D-An elongated bipolar sunspot group with penumbra on both ends of the group; longitudinal extent of penumbra is more than 5°, but does not exceed 10°.
E-An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10° but not 15°.
F-An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15°.
H-A unipolar sunspot group with penumbra. Class H sunspot groups become compact Class D or larger when the penumbra exceeds 5 degrees in longitudinal extent.
p (the penumbra type of the largest spot in the group):
s-small (<2.5 degrees north-south diameter), symmetric
h-large (>2.5 degrees north-south diameter), symmetric
c (the compactness of the group):
x-a single spot