There are concentric and eccentric transitions:
the first are used for vertical pipes, the second - for horizontal.
http://kentreyd.com/en/branches-tees-transitions-g15062475

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Note added at 29 mins (2019-12-11 03:52:05 GMT)
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Observation: The element called transition can also be called a coupling.

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Transitions are eccentric, connecting details of pipelines

Transitions are eccentric. Connecting details of pipelines Transitions are used for connection of pipes of different diameter at construction of the pipeline. Transitions eccentric are applied to interface of pipes on the lower forming line. As a rule, transitions eccentric are applied on horizontal pipelines of power, oil, gas and chemical industry. Design feature of transition eccentric is that its top part on the opposite ends has diameters, different in the size. Application of eccentric transitions gives the chance to prevent formation of stagnant zones in pipeline system. Transitions allow to provide high-quality service of the pipeline, and also carrying out repair of elements of pipeline system, unsuitable for use.


https://kz.all.biz/img/kz/catalog/297562.png
https://all.biz/transitions-are-eccentric-connecting-details...