It is widely agreed that the design of the rotor and wiring system in the enigma resulted in the not so irrelevant error that a letter could not be coded into itself and hence gave a great help in using cribs.

However I thought that the plugboard would eradicate most of this error or perhaps I do not understand it correctly.

As an example assume A is connected on the plugboard to B. Now we know that B will be coded to some letter other than B itself. I assume it could be coded to A and hence pressing key A would light up A.

I would love to know the answer to this.

Regards Trevor Weaver