What letters alliterate?

Humans have it easy. If we want to know whether two words alliterate, we can just listen and decide for ourselves.  It’s poetry, so if it sounds good, it is good.  Computers don’t have that option.  We have to tell it which sounds are equivalent and which are different.  The basics are taken care of by the pronouncing dictionary, but we still have to deal with the edge cases.

In Anglo-Saxon verse, Jun Terasawa is the authority. That’s the scheme I’ll start with. B, D, F, G, L, M, N, P, R, T, and W are in classes by themselves; they don’t alliterate with anything but themselves. The complicated classes are: {G, even when pronounced like Y}; {C, whether pronounced as CH or K}; {S}; {SC}; {SP}; {ST}; and {A,E,I,O,U,Y,H}.

The pronouncing dictionary takes care of G and C.  The letter “S” is different from the three di-consonants it helps form in Anglo-Saxon. (but read on!)  The vowels have a couple of interesting features. Not every case of a word beginning with a vowel alliterates. This came up in the first version of the program: It found the line, “Aragorn and Éomer and Imrahil rode” and said that was a good alliterative line. I put it to the company at Thursday Nights at the Green Dragon, all of whom agreed it was not.  This is because the thing that’s really alliterative isn’t the vowel, it’s the glottal stop when the previous word ends with a vowel, too. (Terasawa, §2.1.) That’s how the “H” finds its way into that group. In a sense, H plays the same role as the glottal stop. We know this is a complicated issue because the English still have problems with words beginning in “H”.

Now off to the computer. We know a syllable has the stress when its vowel has a “1” after it. So when the computer finds a “1”, we can tell the computer to back up to the previous consonant and that’s the sound on which we alliterate – almost. Some stressed syllables begin with two consonants, and we want to alliterate on the first of them. Like “blue-embroidered”: using the naive rule rule would say the consonants to check are L and R, and the computer would return “no”. But those of us with ears to hear would call that an alliteration on B. So we build those in as special cases to test first.  The di-consonants are: BL BR CL CR DR DW FL FR FY GL GR KL KN KR PL PR SC SK SL SM SN SP SQ ST SW TR TW.  When it sees one of these cases, the computer has to back up two consonants to find the beginning of the syllable.

I’m breaking some of Terasawa’s rules since we’re speaking Modern English — I think phrases like “second story” alliterate now, so “S” and “ST” are in the same equivalence class. “S” and “SH” are still different, so “SH” isn’t in the list of di-consonants.

Works Cited

Lerner, Alan Jay and Frederick Lowe. My Fair Lady. New York, 1956.

Terasawa, Jun. Old English Metre: An Introduction. University of Toronto Press, 2011.