I use mainly 40mm but have 30mm and 50mm. So it just depends on the size of the flower or bow I'm putting it on. I bought a whole bunch of 5/8 velvet ribbon from UB and paper mart to line the clips with (I line them pretty much like you line a partially lined alligator clip -Just a little bit on the back, all the way across the top and along the whole inside). I use them with FOE headbands so they sit completely flat and they work perfect!!! My customers love it too because the velvet keeps it in the hair very well too! So it grows with them. The 40mm work perfectly with most of the AR flowers. I have gotten snap clips from both UB and hairparts and I love hairparts square snap clips! They are great!