coupler

small fits small tips

big fits medium tips

big tips wont fit in-- here's a tip:

#1: just put icing in a piping bag then cut the end ("bag A")

#2: get another piping bag, cut the end and insert a big tip ("bag B")

last step: insert the piping bag with icing into the piping bag with tip! (insert "bag A" to "bag B")