Diodes across relay coil

"Diodes are often used across across coil to ..."

A diode across the coil of a relay provides a path for current when the current path to the relay is interrupted (i.e. switched off).  This allows the coil field to collapse without the voltage spike that would otherwise be generated.  The diode protects switch or relaly contacts and other circuits that may be sensitive to voltage spikes.  

Hi JimR, thanks for your detailed explanation. To expand on what little I've written, I've quoted you here:  https://www.the12volt.com/relays/relays.asp

 the12volt