In Star Wars, which is more common, thresh or trill herf?

Question

In Star Wars, the Galactic Basic Standard alphabet has the letters thrill (t) and herf (h) but also the letter thresh (th). Which is more common to use: thresh, or trill and herf?

Answer 1

Typical Alphabets

Normally, in a alphabet that has glyphs for digraphs and dipthongs, you would always use the correct glyph based on how the word sounded. In this case, the Galactic Basic Standard alphabet (called Aurebesh) has thresh to represents the th sound, so you would use that letter when it represents a single sound.

For example, when you pronounce the word "the", you make two sounds "th" and "e". That is a case where you'd use the letter thresh. On the other hand, if you pronounce the word "hathead" you pronounce the "t" and "h" as separate sounds. In that case, you would trill and herf as separate letters.

Aurebesh In Canon

Unfortunately, it doesn't look like the canon examples of Aurebesh we have to work with followed this rule very much, if at all. For example, this message is see on Anakin's speeder in Phantom Menace:

The second line reads "Turn The Ship", and as you can probably see, "the" is spelled "trill - herf - nern" and "ship" is spelled "senth - herf - isk - peth". These are both cases where you would expect "thresh" or "shesh" to be used, and they weren't.

Similarly, this scene is from The Clone Wars:

where you can see the word "threat" (third large yellow word) spelled with trill-herf and not thresh.

Conclusion

Unfortunately, many of the other cases where we see Aurebresh in the movies are just gibberish -- they don't translate into real words that we could tell if the thresh was used properly. Given that the few solid example of in-canon use I can find does not use thresh, I suspect it's probably used very rarely. Thus, it's seems to be far more common to use trill-herf than to use thresh, even in cases where thresh would be correct.

Answer 2

According to Wookieepedia:

It should be noted that in many instances, the sounds "ch", "sh", and "th" are written using Aurebesh exactly as they would be in English (cresh-herf, senth-herf, and trill-herf, respectively), despite that separate letters exist in Aurebesh for those sounds (cherek, shen, and thesh, respectively). [...] While it is possible that these instances represent legitimate in-universe variations of the sound-values for the letters in question, it is more likely that they represent errors on the part of the real-world transliterators (who are understandably more used to employing digraphs than employing single letters to write these sounds).

So at least one (admittedly non-canon) source is of the opinion that the use of digraphs to represent the "th" sound in Star Wars is simply an error. As such, it might not make sense to speak of the in-universe frequency of such usage.

If one wishes to take all instances of this usage as canon, it would probably be impossible to estimate frequency from such small samples. If, on the other hand, it is treated as an error, many of the examples are referenced in the linked article.

Answer 3

Trill Herf is used most commonly, perhaps for the same reason we use TH rather than Thorn (Þ/þ) or Eth (Ð/ð) - the digraphs simply went out of style.

Alternatively, maybe it's for the same reason we have no grapheme Š for SH - it's only used in certain languages/dialects and we've found no reason to adopt it in English.

The first reason makes most sense. Maybe these letters recently went out of use in the Republic.