electrodes

To start with, I have searched the forums for an answer to this. My apologies if I missed a thread on the topic somehow. I was just looking for some clarification on gapping a plug with 2 side electrodes. Is the gap measured from the edge of the side electrode to the side of the center...

Hi Guys, I went to my local auto factors to get a new set of plugs for my 97 aeb engined 1.8t,the plugs that were in the car had two electrodes but the guy gave me a set with 3 on them,he reckons that the car runs better on them,they are bosch part no f7ltcr in my haynes manual it recommends...