Graphs of the Y-Δ transform. Each graph connects the nodes N1, N2, N3; the transform is a simple calculation of the flows through the edges of one graph given the flows through the edges of the other graph. The primary application of the transform is in design and analysis of electrical circuits, but it is a theorem of graph theory, meaning that it can also be applied in other fields. 
This image is meant as a replacement of Wye-delta.svg, with the a/b/c subscripts rotated to more logically correspond with 1/2/3 as suggested here. A new image was created rather than modifying the original because articles in many languages already depend on the notation in the original.
Own work based on: Wye-delta.svg
Xyzzy_n