A comparison of the mean values of the fractional amplitudes of the first three Fourier components of the H i surface density distribution. Upper row:  ⟨ A₂ ⟩  vs.  ⟨ A₁ ⟩  for large and small radii, beyond and within the optical radius, respectively. The  ⟨ A₂ ⟩  values denoting bars and two-armed spirals dominate over lopsidedness within the optical radius, for amplitudes  > 0.2. Thus, strong bars are more likely in the inner galaxy than very strong lopsidedness. Lower row:  ⟨ A₃ ⟩  vs.  ⟨ A₁ ⟩  for large and small radii, the  ⟨ A₃ ⟩  values are smaller than lopsided amplitudes, especially at large radii.