Single Pole Single Throw Relay

Single Pole Single Throw Relay an electromagnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), and one normally open terminal (87). It does not have a normally closed terminal like the SPDT relay, but may be used in place of SPDT relays in all diagrams shown on this site where terminal 87a is not used.

Dual Make SPST Relay  :

Single Pole Single Throw Relay an electromagnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), and two normally open terminals (87 and 87b). Dual make SPST relays (Figure 4) are used to power two circuits at the same time that are normally isolated from each other, such as parking lamp circuits on German automobiles. 

The diagram below center (Figure 5) shows a dual make SPST relay at rest, with the coil not energized. The diagram below right (Figure 6) shows the relay with the coil energized. The coil is an electromagnet that causes the arms that are always connected to the common (30) to pivot when energized whereby contact is made with the normally open terminals (87 and 87b). 
 

Diodes are most often used across the coil to provide a path for current when the current path to the relay is interrupted (i.e. switched off, coil no longer energized). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes. (JimR, contributor, install bay member)