> for example he use the 4066 quad bilateral switch but use only
> one switch the other 3 stay unconnected and of course not used.
> my question is should I follow this order? even in the manufacturer's
> datasheet it's not mentioned.
>
> what should I do? is it correct?

CMOS inputs should be tied to something. If they don't there's a
chance that oscillations can start which may use excessive power
or affect other functions of the chip. You don't necessarily have to
tie them to the power rail, a logic line would be generally OK, even
through a high (100k) resistor

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