Smooth-armed spirals as a class were first introduced by [177]; see also [178] and [207] for a preliminary list. Later, [35, 38] the systematic variations in the orderliness of spiral arms were categorized (see the table below); broadly considered, Arm Classes 1 through 4 were termed flocculent, while Arm Classes 5 through 12 were said to be grand design. Discussions of Arm Classes and their correlation with other parameters can be found in [33, 34, 36, 37, 47, 124, 147, 152, 175]. Arm widths are discussed by [15].
| Arm Class | Description | Prototype |
|---|---|---|
| 1 | Chaotic appearance; no symmetry; fragmented arms with differing pitch angles. | NGC 2500 |
| 2 | Short, fragmented spiral arm pieces defined mostly by HII regions; no regular pattern; loose, open arms. | NGC 7793 |
| 3 | Fragmented arms, uniformly distributed around the galaxy center. | NGC 2841 NGC 5055 |
| 4 | Only one prominent arm; otherwise fragmented spiral arms with no regular pattern in the rest of the galaxy. | NGC 2403 |
| 5 | Two symmetric short arms in the inner regions; irregular arms in the outer regions. | NGC 1313 NGC 0598 |
| 6 | Two symmetric arms; feathery appearance in the outer arms, which tend to wrap tightly and form a ring-like structure. | NGC 2935 NGC 1637 |
| 7 | Two symmetric, long arms in the outer regions, and irregular or feather arms in the inner regions. | NGC 2903 |
| 8 | Tightly wrapped arms forming a ring-like structure with over-all symmetry. | NGC 3992 |
| 9 | Multiple, long and continuous arms in the outer parts, two symmetric and continuous arms in the inner parts. | NGC 5457 NGC 1232 |
| 10 | Two long arms extending from either end of a prominent bar (abandoned in later applications [38]). | NGC
1300 NGC 5383 |
| 11 | Two long symmetric arms in the inner regions with a long drawn-out arm or arms in the outer regions; near-by galaxy apparently interacting (abandoned in later applications [38]). | NGC 7753 NGC 5194 |
| 12 | Two long, sharply defined symmetric arms dominating the total appearance. | NGC 4321 |