How to connect a net with many pins using the least total wire. Routers split a multi-pin net into 2-pin connections using these.
How it looks
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Net Net-(Q1-B) on CORE3588 (amber): three pads joined as a tree. Other nets grey.
1Q1 pin 1 (the transistor that switches the red RECORDING light).
2R60 pin 2: the routed copper uses this pad as the branch point, so the three pads form one tree with no loops.
3R59 pin 2.
Real pad positions from CORE3588, tracks hidden, spanning tree computed for this card
The reset-button net (six pads around SW1), copper stripped. The dashed lines are the minimum spanning tree: the shortest set of straight links that joins all six pads with no loop (five links for six pads). This is what KiCad draws as the ratsnest.
How it works
Where it fits
The MST is what you see as the ratsnest: each dashed link becomes one 2-pad job for the search (A* or Lee).