Trees grow towards the light.  In the city, that means growing away from the shadow of buildings, leaning out at an angle that become more precarious as the weight of branches increases.

Pruning the crown of a tree can help to rebalance it but branches will regrow quickly on the sunny side.  Cautious tree managers may decide that a tree is too unbalanced to be safe. Sometimes they may give it some support instead.

This robinia tree stands just outside the Musée des Arts et Métiers, in a corner between the former church of St Martin and the main museum building.  It probably owes its survival to some imaginative engineers.