It should also be noted at this point that the machine is reciprocal, that is to say that, if at a given position of the machine N lights up A, then A will light up N.
Each time a key is pressed the right hand wheel moves on one so that if, in the position immediately following our example above, the same key is pressed the current will enter the right hand wheel at B and not A and will pursue an entirely different course. Once in every 26 positions the right hand wheel moves the middle wheel over one so that when the right hand wheel returns to position A, the middle wheel is in a new position. Similarly the middle wheel turns over the left hand wheel once for every complete revolution it makes. Thus it will be seen that 26 x 26 x 26 (about 17000) letters have to be encyphered before the machine returns to the position at which it started.
For most of the period with which we are concerned there have been 8 wheels. Wheels 1 to 5 turn over the wheel next to them once per revolution, wheels 6, 7, 8, twice per revolution, this somewhat complicating the cycle of the machine as described in the previous paragraph. The turnovers (by which I mean the position of the wheel at which it turns over its next door neighbour) on wheels 1 to 5 are all in different places, in 6, 7, and 8 they are always at M and Z. As we shall see later this was an important development.
On each wheel is a tyre, marked with the letters of the alphabet. One of these letters can be seen through a window on top of the machine and the position of the wheel is referred to by the letter shown in the window. The tyre is completely independent of the core of the wheel which contains the wiring, the relative position being fixed by the Ringstellung or clip which connects the tyre with the core. Thus even if the starting position of the message is known, it still cannot be decoded unless the clips, which fix the relative position of tyre and core, are known also.
The Enigma machine would be a comparatively simple affair if it were not for the Stecker. This is a substitution process affecting 20 of the 26 letters before and after the current travels through the wheels. Let us return to our original example and assume for the moment that A is steckered to F and N to T. In our example we pressed key A and entered the right hand wheel at a position we called A but if we now press A the current will be sidetracked before entering the wheel and will in fact enter at F and pursue a quite different course. If on the other hand we press F, the current will enter