The Baldwin-Phillips-Terlevich diagram (Baldwin et al. 1981) for emission-line galaxies. The emission-line galaxies observed in this paper are shown by filled blue circles and the most-metal deficient star-forming galaxies known (Table 10) are shown by red filled circles, including observations of several knots of star formation in SBS 0335–052E. The Local Group dwarf irregular galaxy Leo A is encircled. Also, plotted are the 100 000 emission-line galaxies from SDSS DR7 (cloud of grey dots) and a sample of 803 luminous compact galaxies (LCGs, open black circles) (Izotov et al. 2011). The dashed line from Kauffmann et al. (2003) and the solid line from Stasińska et al. (2006) separate star-forming galaxies from active galactic nuclei. The latter lie in the right wing of the “butterfly”.