SoftwareSerial missing from library

I'm working with an RS485 and the code I have found uses the SoftwareSerial library. It seems to be a standard Arduino library, but for some reason it is not in mine. I have searched everywhere and even installed the latest version of the IDE, but it didn't help. Any help would be welcome. Thank you in advance.

What version of the IDE are you using ?
How have you established that the SoftwareSerial library is missing ?

I use Arduino 1.6.12
I checked in Sketch >> Include Library >> Arduino libraries and it is not in the list.
when I include it in my sketch I get the following error:

fatal error: SoftwareSerial.h: No such file or directory

#include <SoftwareSerial.h>

^

compilation terminated.

exit status 1

I know now what the problem was. I was using an Arduino Due and it does not seem to support the SoftwareSerial library.