We present a study of pattern formation beyond the onset of the wave instability in a short model reaction-diffusion system whose length is between 0.5 and 1.5 times the characteristic wavelength of the wave instability. As the system length is varied, modulated standing waves, characterized by short-lived alternating nodes, are found between the domains of the half-wavelength and the one-wavelength standing waves. The space-time two-dimensional Fourier spectra of these modulated standing waves show large side peaks. The position of these peaks differs from that of the fundamental peak by its wavenumber and by the frequency of appearance of the alternating nodes. Another region of modulated standing waves is found within the domain of standing-traveling waves.