Before it was established that there was a triplet, non-overlapping, comma-less genetic code* for what at that time were thought to be only 20† amino acids, there was uncetainty about:
The codon size (must be more than 2, but...)
Whether the codons overlapped (e.g. in a stretch ABCDEFG with triplet codons, whether ABC, BCD, CDE etc were the codons)
- Whether there were ‘commas’ as demarcation betwen different codons
These and other problems were disscussed by Crick et al. in Nature (1961) 192 1227–1232. There is an introductory gloss of this on Scitable, with a link to free-view of the original paper.
*We now know there are variants on the standard genetic code, but all the codes have the characteristics mentioned.
†In addition to the standard 20 amino acids stop codons can be subverted in certain circumstances to encode selenocysteine and, in some bacteria, pyrrolysine.