I love these holders, however I did not realize that a bottom/side railing was needed to lock in the planter.  The picture does not show the extended piece that needs to rest on a bottom rail.   All is well because I just used two screws to fasten it to the back of the top railing and it is very secure.  I planted a trailing mandevilla and  it will look so pretty trailing out of the pot.  I was able to  put together and fasten both planters within 30 minutes.