I've used the following from most frequent to least.
1) Leaving the sangles loose but UNDER the flap and utilizing the turnlock to secure closure.
Be mindful that sangles in a stiffer leather can cause the flap corners to curl upward!
Because of this, I also do option 2:
2) Leaving the sangles loose, but seated on TOP of the flap (still utilizing the turnlock to secure closure)
You can still open the flap, but just a little more effort to pull it over the floppy sangles.
I enjoy this look because I can enjoy the aesthetic of the sangles (which is what drew me to the Kelly and its siblings in the first place), but still have reasonable access to the interior.
3) Securing the sangles underneath the flap and utilizing the turnlock.
Easy to open the flap, and can works well if you like to use the slip pockets in the front, such as with a slim cardholder, passport, or boarding pass. Not as easy to grab something like your book or wallet because you may scuff the interior - the opening remains tight while the sangles are secured.
You can undo one or both of the sangles to loosen one the sides, depending on what you're trying to wrestle out.
4) Securing the sangles and tucking in the flap behind the gussets like a Birkin.
Again, good to get a slim card holder or other paraphernalia from the slip pockets. I'm not fearful of weight distribution because the flap sits in a lovely manner between the gussets of leather.
I stopped doing this for fear of scuffing the front flap, but I must say - pretty convenient.
Shows off the clean and architectural lines of the Kelly in a different way, but ultimately convinced me to seek a HAC/Birkin.
5) Leaving the Kelly completely open - sangles to the wind and flap dangling like a loose tooth.
(Regarding the traditional Kelly: I know that the "carefree" it-girls do this. But to me, I wouldn't recommend; to me, it blurs the line between "carefree" and "careless." Strain is introduced to the back panel of the bag unnecessarily, and frankly, it looks sloppy.)
That said, in the Kelly messenger's case, since the weight is distributed along the sides of the bag's body, I could see leaving it completely open as a viable option.
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I think the Kelly messenger bag is a welcome addition to the Kelly line for its casual, masculine, utilitarian, and sporty traits.
As I've mentioned on other threads, I feel that the strap placement is a bit strange. Seems as though it may be top heavy and could lead to tilting and flipping. I also don't appreciate that the strap is non-detachable.