Reading the Wikipedia page for SNPs I find the sentence "Almost all common SNPs have only two alleles." This is consistent with terminology elsewhere, such as the therm "Minor allele frequency", which references the least common allele, which implies there are generally only two alleles for a given SNP.
But there are 4 common nucleotides, A, T, C, and G. Why wouldn't it be common to have 3 or 4 alleles at a certain location? The only way this seems possible to me is if strand is ignored, and so at any given location "A" and "T" are considered the same allele and "C" and "G" are considered the same allele. Is this the case, or am I misunderstanding things.