HOW UNFORGETTABLE?: <i> How could a song from 1951 be declared the best song of 1991?</i>

That’s the question everyone seems to be asking in the wake of the Grammy victory by “Unforgettable,” which was written by Irving Gordon 41 years ago and re-recorded last year by Natalie Cole.

The answer: Under National Academy of Recording Arts & Sciences rules, any composition is eligible in the “song of the year” category as long as it was recorded during the eligibility year and was not previously nominated in the same category.

The strongest defense for the rule involves the case of the “overlooked song”--a song that appears on an album that receives almost no national attention, only to be turned into a hit by someone else a year or two later.

But “Unforgettable” was hardly overlooked. The ballad made the Top 20 twice--once via the original Nat King Cole recording and again in 1959 via a Dinah Washington recording.

The tantalizing question is whether “Unforgettable” would have likely been nominated for best song in 1951 if the Grammys had been around (the awards didn’t start until 1958).

The answer: Probably not.

Given Grammy voters’ preference for mainstream bestsellers, the song--which only made it to No. 17 on the weekly Billboard charts--would likely have been overshadowed by numerous mainstream hits that year, including these five:

“Cold, Cold Heart.” This Hank Williams ballad, which remains a staple in country music, was such a sweeping hit that five different recordings of it reached the pop Top 30, including Tony Bennett’s version, which spent six weeks at No. 1.

“Too Young.” This Sylvia Dee and Sid Lippman ballad about teen-age romance was a far bigger single for Nat King Cole than “Unforgettable.” The record spent five weeks at No. 1.

“Because of You.” Tony Bennett’s recording of this soaring 1940 ballad by Arthur Hammerstein and Dudley Wilkinson spent 10 weeks at No. 1 and would certainly have appealed to the Grammy Old Guard.

“Cry.” Some rock observers point to Johnnie Ray’s ultra-dramatic rendition of the Churchill Kohlman song as one of the key links in the evolution of R&B-pop-rock.; The record was No. 1 for 11 weeks.

“Come On-a-My House.” Don’t scoff at this Ross Bagdasarian/William Saroyan novelty. Recorded by Rosemary Clooney, it was the nation’s No. 1 single for eight weeks. Grammy voters in the early days of the academy frequently went for left-field entries. In fact, Jimmy Driftwood’s country-folk novelty “The Battle of New Orleans” was named best song in 1959, the other year that “Unforgettable” was a hit.