On the subject of ‘through’ and ‘thru’

A co-coworker came to me today and asked my opinion about using ‘thru’ vs. ‘through’, and when I did some looking, I found that the definitions listed in most dictionaries appear to be missing an important consideration (in fairness, dictionaries are not generally repositories for the finer points of things like this).

‘Thru’ is, at least according to most of the dictionaries I consulted, an accepted variant spelling of ‘through’, but is noted as being considered “informal”. I would agree with that for most prose, but there are cases in which ‘thru’ is, at least to me, perfectly warranted even in a semi-formal writing.

The main usage that comes to mind, and the one the question was about, is using it in the context of describing a range, in which case I feel that the most appropriate variant is the one that most closely matches “how compact” the overall expression is. For example:

• Tuesday, July 4^th through Friday, July 7^th
• July 4^th thru 7^th
• July 4^th through July 7^th
• July 4–7 (that's an en dash, for the heathens out there…)

Thus, using ‘thru’ is appropriate as a “mid-point” between the longest form (the word ‘through’) and the shortest (the en dash).