isotope standards.

hi, this is a purely theroretical query (possibly answered with a simple reference i'm just curious about the histroy and whether there was any thought behind it or if it was simply a case of eeny meeny miney mo.
the isotopic composition of a sample is often expressed as a delta notation relative to the isotopic composition of a standard (e.g. d13C in 0/00)
the standard typically used for carbon is the Pee-Dee Belemnite (PDB). Now this limestone has a relatively high 13C/12C ratio, therefore values of inorganic or organic carbon are mostly negative, i.e. isotopically lighter than the PDB standard.
can anyone tell me why we use the PDB? and also if other standards used (i.e. for other isotopes) also have relatively high or conversely relatively low ratios?

thanks

Lys