I think it is fine to use "ITB" with other DGs that you haven't met. After all, it is that bond of sisterhood that keeps us together.
I have used it with DGs that I haven't met...both DGs who are in their 20s and DGs in their 50s and 60s!
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By the way, here is something important that I found...sometimes, I have seen some DGs sign off "itb" (i.e. lower case), and this is actually incorrect (I think I am guilty of it myself!
)
Anyhow, here is the 411 on the right way to write "ITB"...
ITB - Always capitalized; when spelled out, capitalize "I" and "B" only.
Source:
http://www.deltagamma.org/resource/terms.htm#letteri
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