"Wobble"
Pairing of the tRNA anticodon with the mRNA codon proceeds from the 5' end of the codon. Once the first two positions are paired, exact base pairing of the third position is less critical. The third (5') base of the anticodon can typically pair with either member of the purine or pyrimidine pair in the codon as appropriate: it "wobbles". In this example, the double-ringed G can pair with either a single-ringed U or C. This allows mRNA to be translated with fewer than the 64 tRNAs that would be required without wobble. Some wobble positions can pair with any of the four bases.