In most of the cases, chances are that your watch already has spring bars in it that were used to attach the OEM strap or bracelet. We recommend reusing those as you can be sure that they are the perfect size for your watch.
But it is highly recommended for the customer to take them out and check them for being bends, faulty spring(s) or any other defects. Lastly check if they are 'quick release' type. Such spring bars are integrated into the strap and should not be reused in another strap. In any of the above cases, it is recommended to add a set of spring bars to your order for a seamless experience. Here is an example of quick release spring bar.