what does double pole double throw mean?

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The following list explains the differences between the Von Duprin 47 and 48 latch assemblies. The 47 top and bottom latches have 5/8" throw latchbolts. The 48 top and bottom latches have 3/4" ...
The standard latch projection is 1/2".  Falcon also offers a 3/4" throw with a 2-3/4" backset.
The ND-Series RX,  the L-Series RX and latch monitoring switches are Form C - Single Pole Double Throw (also known as SPDT).
DPDT is double contacts push bar monitor switch. Double Pole Double Throw
The Request to Exit (RTX) switch input can be used as a remote door release.
A single-pole, single-throw switch (e.g., pushbutton switch) must be connected across the RTX input pins.
When the switch changes state (e.g., from open to closed), the CT5000 will change the state of the Strike relay.
The AD and CO locks both have a 3 hour fire door rating. Please see URL for more info.  UL Fire Rating List
Double cylinders are only available on Von Duprin 98/99 Rim and 98/9975 Mortise devices.
The Schlage ND is shipped with a driver/tailpiece when ordered with a LD, BD, or JD suffix.
EPT can be used on following hinges: Degree of the opening Hinge type Door thickness 0-180 degree Up to 5" butt hinges 1 3/4" 0-180 degree Up to 3/4" offset pivots 1 3/4" 0-130 degree 5 1/2"...
See related solution: Can an EPT be used with Cam Lift Hinges?