Rule stating that the cosmic abundance of elements with an even atomic number is greater than that of adjacent elements with an odd atomic number. Consequently, a graph plotting relative atomic abundance against increasing atomic number (Z) displays a ‘toothed’ curve, rather than a smooth line. The reason for this is connected with processes such as helium burning (see nucleosynthesis). ⁴₂He is a basic building block, and so additions produce even numbers, e.g. ⁴₂He + ⁴₂He → ⁸₄Be; ⁸₄Be + ⁴₂He → ¹²₆C.