Some telescopes only have a T-threading upon the rear, instead of a 1.25" or 2" inner diameter focuser barrel.  The Lumicon T-Mount Female to 2" Female Eyepiece Adapter allows for 2" eyepieces and accessories to be attached to such a scope.  The front (where light enters) has a female T-thread, and the rear (where light exits) has a 2" inner diameter barrel with a set screw to affix your 2" accessory.

Other applications for this fitting include to use 2" eyepieces on Lumicon's 1.25" Multiple Filter Selector & Solar Prominence Filter, or to use 2" eyepieces on Lumicon's 2" Multiple Filter Selector when the LF5020 photographic adapter's installed.