The choice of frequencies for SETI has been discussed by many authors. We wish to point out that the line spectrum provided by the hydrogen (H) atom is very rich, and covers a large spectral domain, from the ultraviolet to radio wavelengths. The frequencies of H lines present one remarkable feature, which in our opinion makes them more attractive than those of, e.g. the OH or H2O lines, or those chosen arbitrarily in the water hole: they all relate to only one atom, the simplest and most abundant one in the universe. In terrestrial alphabets, one word is composed of one or several characters. We can always replace any set of characters by a set of H frequencies, and “write” messages with them. Since the H frequencies are universally known, a “cosmic alphabet” could be constructed with them. In a mono-frequency search for artificial radio signals, the frequency of the H166 alpha line, 1425 MHz, might be a good substitute for the frequency 1420 MHz of the H 21cm line. In a multi-frequency search, the frequencies of the prominent Hn alpha lines near H 21cm might be interesting. There are about ten of them in the water hole. Finally, owing to the physical link between them and the H 21cm frequency, those around this symbolic frequency appear most attractive.