Diode
A Diode is a two-terminal passive electronic component exhibiting non-linear behaviour. In its ideal form, it allows current in a unidirectional flow, blocking it completely in the other, similarily to a check-valve.
It is used to perform various advanced functions when placed in the feedback path of an Open Amplifier, which are beyond the scope of this introductory page.
While in the heyday of analog computers diode could be implemented as vacuum tubes, on The Analog Thing silicon diodes are used, of two different kinds: Rectifier Diodes (Four of them are present on THAT) and Zener Diodes (There are 2 of these). Further information on Z-diodes can be found here.
The Rectifier Diodes in THAT are 
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Physics of Diodes
Diodes in Analog Components
Diodes are used in a multitude of different circuits (Precision Limiter, Bang-Bang, Dead Zone, DFGs)
