Threadolets are generally easy to install and work with, depending to some degree on the scale and scope of your project. These olets are easily fitted onto connecting pipes to help create a smooth transition between them. General olet fittings provide an outlet from a larger pipe into a smaller pipe or a pipe of the same size. The attached video will walk you through the correct application and use of threadolets.
The Weldolet is the most common of the branch connections, and are designed to help minimize stress contractions by providing solid reinforcement.
It is welded to the outlet pipe when installed correctly. The ends of this olet are beveled to help make the welding process easier, thereby making it a butt-weld fitting.
The thredolets use the Weldolet, however the branch of the Threadolet affixes through the use of a thread inside of the olet. The bore here matches the outlet bore, and the existence of the threading means that no welding is necessary, as the thread facilitates installation.