Have you ever needed different stepping adapter sizes which are not available? Have you ever wanted a female to female thread adapter to join two pieces of camera equipment together? If so, read on. You may be surprised that what you needed before is actually right under your nose; all it takes is a little ingenuity.
There are some step-down rings which have larger male thread size (usually 55 to 67mm), with smaller female threads (perhaps 49 or 52mm) that extend the entire thickness of the ring. This can be effectively used as a female to female adapter e.g. 52 to 49mm step down ring will have 52mm male threads on one end and 49mm female threads which span the entire thickness.
With a few of these adapters, you could make your own custom setup. You then have two sets of threads to work with, to mount different items to it e.g. lens hood, filters.
Another very useful stepping ring is the M39 to M42 step-up ring. It allows the use of M39 enlarger lenses with M42 focusing helicoids.