Wedding Dress, Bridesmaids Dress, Mothers Dress... Rehearsal Dress?

Obviously I knew about my wedding dress. I also knew about bridesmaids dresses, and mothers dresses, but a rehearsal dress? I have seen some posts lately about the dresses that girls are going to wear to their rehearsal too. Such as

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I truly have never thought about a dress I would wear to my rehearsal. While it would be really fun to wear a dress like this:

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I think it would be out of place. Is the rehearsal supposed to be somewhat dressy? We were going to go with a little bit of a more casual comfortable feel, thinking BBQ at the in-laws! So is it ok if we just wear jeans and shirt? Or should we dress it up a little bit more?