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Galaxies with grand design spiral patterns sometimes have optical features that are associated with resonances. These include the outer Lindblad resonance at the outer edge of the spiral, corotation at the ends of symmetric bright star formation ridges, and interarm spurs at the inner 4:1 resonance. The ratios of these resonance radii for about 20 galaxies that have been studied are consistent with predicted ratios based on the rotation curves. Some galaxies also have symmetric 3-arm spirals between the inner and outer 3:1 resonances, in addition to the more obvious 2-arm spirals.
M81 is one of the few cases found so far that has symmetric spiral arms plus a symmetric variation with radius in the spiral arm amplitude. This radial variation could be an interference pattern such as that predicted by the modal theory of spiral structure.
Most galaxies with long spiral arms have 2 bright symmetric inner arms. The inclination-corrected lengths of these inner 2 arms were measured for 40 SA, 80 SAB, and 40 SB galaxies. Correlations of the 2-arm/bar ratio and the 2-arm/R25 ratio with Hubble type and absolute magnitude suggest that galaxies with big bars have their bars ending near corotation and their 2-arm spirals ending near the outer Lindblad resonance. Galaxies with small bars or no bars have their bright 2-arm spirals ending near corotation, and weaker or multiple arm spirals extending to the outer Lindblad resonance.
