I love how everyone seems to think it's a purity ring. Not synonyms people. Guy above quoting wiki has it right.
Either way, the only girl that got a promise ring from me I haven't talked to in 3 years so...
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Livewire Wifey: Katie :D
Windows, For when you don't know any better.
Mac, for when you think you do.
Linux, for when you actually want to get something done.