We established M51 systems as a class of interacting pairs of galaxies in 1959. in 1975 I showed that among 160 systems with this appearance the ratio of luminosities (and hence masses) of the components varies from 1:1 to 1:0.01. in the latter case, the companion is comparable in mass and dimension to an isolated H II region. the number of such M51-type systems increases as the luminosity of the companion decreases. Small companions cannot draw spiral arms from the primary companions by means of tides. Yet, they are observed at the tips of spiral arms. Thus, they must originate at the same time as the spiral or form within them. in some cases, these companions emanate from the spiral arm, as is shown by comparison of observations and calculations.