Warning: Simplified (and smarter) spelling ahed!
Lately I'v run up against the thru / through debate again. It mainly swirls about whether thru can be noted formally, that is, whether it can be written in academic papers, newspaper or magazine writs, or in books. The short answer, without nay, is YES!
Let me say at the outset, that this is mainly for those who write in the style of American-English (AmE) rather than British-English (BrE). The British think of thru as an Americanism and likely heretical; the Oxford Dictionary Online labels it as chiefly N. Amer. and, not amazingly given their slant against American fonetic spellings and that they like their French rooted spellings such as -ough cluster of words, tags it informal. (Merriam-Webster does not tag it informal). As far as I know, thru is seldom seen, even informally, in BrE tho, I’v read, it is at times seen from American companies doing business in Britain.
I should also say that this goes for the other smarter spellings like altho, tho, thoro, thoroly and all the sundry spinoffs made with them.
Now let’s get to the nitty-gritty. I take most tags, like informal, non-standard, asf with a grain of salt. After all, they can shift at the whim of the editor. The proof is the pudding … the note of the word.
Why would anyone want to write thru in the first place?
1. There is no nay that thru is fonetic whereas the -ough variant is not. The -ough cluster has no less than eight ways to say it.
2. There is no nay that thru is shorter and cleaner.
3. There is no nay that thru is more etymologically sound (see the after-writ below if you don't believ that).
4. There is nothing elegant or even smart about the -ough variant; indeed, many find it ugly and I even call it a stupid spelling which riles up a few folks.
However, there are clerisy snobs, made up mainly of gatekeepers (editors, publishers, and often teachers) and pedants who cling to to these stupid spellings. That, in itself isn’t bad. I truly don’t care if they want to type more letters. But what is bad is how they gnash teeth, wail and moan about thru. Others get downright snitty. They not only snarl, rant and rave against thru and but also often against the folks calling them lazy, illiterate, asf.
The -ough variant of thru is one of the snob spellings; it is a shibboleth. Since the -ough givs no clue to how it is said and since it is needlessly long, then the -ough variant must be memoriz'd. It’s one brick in the barrier of written words that the clerisy snobs believ not only sunders them from the herd but also sets them abuv it. For them to yield to it would lower their haughty, self-perceiv'd status. This is eathly seen on the net nowadays. Often in the comments below a writ the folk write with thru. But for a gatekeeper, pedant, or one of the clerisy to do that … well, they shudder at the thought. They can’t bring themselves to write thru as that would, in their eyes, be beneath anyone of learning and knowledg. It must be said that not all of the clerisy are snobs. For many of them, the spelling thru recks not. But not so for the snobs.
Before the worldwide web, the gatekeepers could keep a tight lid on the herd. In his forward to The Sources of Standard English (1873), Thomas Laurence Kington-Oliphant wrote:
The printers have been good enough to let me write rime in the English, and not in the Greek, way. But I may mention that they have in general struck out z in favour of s; thus they have printed civilise instead of the civilize I wrote. ... I give this as an instance of the shifting that may be remarked in the history of the English tongue: some change or other is always at work. Caxton and his sons have ruled our spelling for the last four hundred years; …
And so it was until worldwide web freed the folk from the gatekeepers and this has made for an almost hateful backlash from many of the clerisy snobs. They often strike back in the only way left open to them … scolding, browbeating, and bullying. They come out guns blazing even saying that it is slang or even it’s not word. Well neither of those hold up at all so they fall back on either it’s informal or it’s non-standard therefore it shouldn’t be noted in any kind of formal writing. None of which are true.
One debater, grasping at straws, even tried to say there can only be ONE formal shape of word and therefore all others must be informal. This is witless nonsense. There has never been any such rule and doesn’t hold up to scrutny anyway. We need only to look at the words judgment, judgement. Both are fully acceptable in any kind of formal writing.
It is eathly shown that thru has been noted for over 100 years in formal works. Before we stroll thru byspels (examples) of such works, let’s stop for a bit and take a look at how this began.
In 1876, the American Philological Association (APA) took up 11 spellings, and began touting them: ar, catalog, definit, gard, giv, hav, infinit, liv, tho, thru, and wisht. Two years later the Philological Society of England (PSE) joind the work. By 1886, the list had grown to 3500 words.
In 1898, the (American) National Education Association (NEA) began touting a list of 12 spellings: tho, altho, thru, thruout, thoro, thoroly, thorofare, program, prolog, catalog, decalog, and pedagog … all of which are still found today. That’s right folks, the National Education Association at one time fully backt these spellings. When and why they backslid, I do not know.
All three groops (APA, PSE, and NEA) gave thru and others their full backing. These words were in no way taggd as informal only. They were meant to fully be noted insted of the other variants in every way. Thus for over 100 years now, thru been noted in books, academic papers, and writs of all kinds. We can scan thru history and find many such times.
1910, Handwork in Wood, William Noyes. This was book “intended primarily for the teachers of woodwork”. Here we find not only thru but also altho, tho, thoroly, and thruout: Then with a hastening rush the top whistles thru the air, and tears thru the branches of other trees, and the trunk with a tremendous crash strikes the ground. (p11)
1915, The Michigan Technic - Volumes 28-29: So we will have the discharge taking place thru an orfice. (p53)
1920, The Modern English Verb-Adverb Combination, Arthur Garfield Kennedy: About, across, around (or round), at, by, thru and with have, as a rule, about the same meanings in combination that they possess as prepositions. (p19)
1930, Story of the Drama, Joseph Taylor: It tells the complete story of the drama in a scholarly yet simple way from its origin thru its evolution, analyzing each phase of life and civilization presented thru the medium of drama down the ages, to the period of the Commonwealth.
1990, Operator's, Organizational, Direct Support and General Support Maintenance: Excess returns to tank thru secondary bypass line 7.
2007, Sideways Thru Time, Frank Menser
2010, Code of Federal Regulations, Title 32, National Defense, Pt. 700-799, Revised as of July 1 2010: … may be obstructed at the following angles relative to the LCAC's heading, from 37.00 degrees thru 90.00 degrees …
So, we can see that thru has been noted for over 100 years—and is still—noted formally in the US.
So why this latest spewing of frowardness, often hatred, by the clerisy snobs? After a littl delving, I found that the writing of thru, in books, hit a low spot about the mid-90s. Likely the clerisy felt that they had held the spred of the word to small things like drive-thru at restaurants, at times a letter to the editor, and writing in the police, military, or technical fields which they grudgingly yield as “special uses” that, for som unknown reason, cannot be noted outside of these narrow confines. That is, of corse, bunk which only highlights the weakness of their argument, that is, it can be noted formally only in “special” ways.
However, about this time, the web truly came into its own and the blogosphere burst forth. No longer did the folk need to go thru a gatekeeper. Notwithstanding that many folks make a good income from blogging, the clerisy at once steppt forth and tagg'd all such writing as informal. After all, in their eyes, if there is no gatekeeper, then it must be informal. Not true. Many blogger write amazingly well and their writs are as good as any writ found in a newspaper, magazeen, or any other work that might be thought of as "formal".
I don't think that it is the blog-world itself that bothers the clerisy snobs so much as they think of bloggers as the herd rather than the clerisy. But the blog-world is a token of a rising groundswell and this time it a bottom-up swell rather than top-down when the National Education Association first put its list out and the note of thru in books has been steddily climbing. This has likely caught their eye and worries them.
And worry'd they should be. They may not yet know it, but they’re alreddy fighting a reargard action. It won’t happen anytime soon, but the thru does not “hurt the eyes” of most Americans. We see it daily from drive-thru to thru-hiking. We see it in sundry shapes thru, 
My rede to those who wish to write with thru in a “formal” writ is to let the gatekeeper know ahed of time. Thruout my university years both as an undergraduate and graduate student, at the outset, I let my teachers know that wrote with altho, tho, thru, thoro, asf. Somtimes I would get a frown and even a slight … tsk, tsk. I think only once did I hav to pull out my pocket wordbook and say, “They‘re in the dictionary if you want to see them.” They knew they had no grounds to refuse me … if one had tried, it would hav gone at once to the dean’s offis. So, knowing beforehand that they would see these spellings, it was no problem.
At the Sheriff’s academy, we were told to write thru on all our reports.
In the corporate world I met no resistance whatsoever when writing formal reports with thru.
I hav also found that at times it helps to do as I do on this blog … put a warning that smarter spellings are to be found in the writ. Then folks will know beforehand that they will see simplified, cleaner, and smarter spellings.
After-writ – The history of the thru.
Many folks say they write with the -ough variant owing to it is “traditional”. They hold the mistaken belief that this is how the word has always been spelt. Professor Walter Skeat, writer of An Etymological Dictionary of the English Language, once said:
It is surely a national disgrace to us, to find that the wildest arguments concerning English spelling and etymology are constantly being used by well educated persons, whose ignorance of early English pronunciation and of modern English phonetics is so complete that they have no suspicion whatever of the amazing worthlessness of their ludicrous utterances."
And so it is with the -ough variant of thru. For true, it is so with the whole -ough cluster of words. First let’s look at how this cluster came to be. In OE, thru was spelt þurh ( þ=th ). Liken German durch. It wasn’t until later that the r and u swappt spots … which often happens in words.
So whence the -ough? After the Norman-French Takeover in 1066, the French scribes began shaping English words to fit French spelling ways. The u was shifted to the French ou (liken OE wund, wunde to today’s spelling of wound [harm, injury]); the h was shifted to gh. Their reasons for doing so were weak and not well thought out so we don’t need to delve into them. Thus, the truth is that the -ough variants are mongrels; they’re nothing more than French bastardizations of English.
There was no royal decree that went out to set the spelling of these words ... William was French and spoke French as did most of the nobles to whom he handed out land. He didn’t care. But the French scribes did hav to work with the English speaking folk of the land. As time went on, their bastardize spellings took root. That’s not amazing given that French was spoken and written by the hierarchy. Given the sundry dialects, it came out a true mishmash:
tho - OE - þêah I. (ê) conj. and adv.; ME also thou(g(e, thouh(e, thouhg, thouth(e, thouw, thouf, thuf & tho, þho, thoch, thof, thof(f)e, thog, ðhog, thogh(e, þhoh, thowe
thru / thoro ... OE - þurh ME - thurgh (prep.) Also thurghe, thurght, thur(g, ðhurg(e, thurgth, thur(o)we, thouro, thoru(e, thorug, thorugh(e, thoruh(e, thorough(e, thorouh, thor(r)ou, thorogh, thoro(g, thor(r)owe, thorgwe, thorw(h)e, therwe, toru & thrugh(e, thrught(e, thru(g, throu(gh(e, throuh, throgh(e, throth, throwe, threu, threwe ...
By the way, thoro is only a later, otherly shape of thru; it was spelt þuru as early as in Havelok, 631, and þuruh In the Ancren Riwle, p. 92, 1. 17. ( þ=th )
So, you can see, there is no true historical or etymological basis to call the -ough variants “traditional” even if this were a good excuse—Does any still write queene? The -ough variants were only one of many variants until the printing press came and Caxton printed the first book in England with it. He pickt the spellings that he lik't and as Kington-Oliphant wrote: Caxton and his sons have ruled our spelling for the last four hundred years; …