How do you measure sizes in girls jeans?

[xiaofen]

I am looking to buy my girlfriend some jeans for christmas, but the jeans i am looking at are measured in a different scale. Usually she wears a 7-9 depending on the jean style. These jeans are from 22-29. How can i find the correct size of jeans without having her try them on. And how are girls jeans measured? I am looking at some jeans if that helps at all... Thanks

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