Diodes between rails
The BAV 99 is two diodes connected in series (cathode of one connected to anode of the other).

I think  you are referring to it used like this:







simulate this circuit &ndash; Schematic created using CircuitLab

With the diodes oriented this way, they will not conduct as long as the signal remains between Vcc and Gnd.  If the signal goes more positive than Vcc + 0.7 volts, then D1 will conduct, preventing the signal from rising above Vcc + 0.7 volts.

Similarly, if the signal tries to go more than 0.7 volts below ground, D2 will conduct.

This arrangement is used to ensure that inputs to an IC do not go outside the safe voltage range of the IC.