In coincidence measurements - such as in photodouble ionisation - one needs to ensure that two (or more) particles are associated with the same physical event. In photoelectron-photoelectron coincidence spectroscopy the two ejected electrons are time correlated in the following manner: An electron arriving at one detector starts a ‘clock’ and the next electron arriving at a second  detector stops it. This is repeated and a distribution of time intervals is accumulated. If the two electrons are from different atoms then the distribution of time intervals will show no correlation, resulting in a flat background of ‘random’ counts. But if the two electrons are from the same atom then a specific time correlation can be expected, resulting in a ‘true’ coincidence peak. In our experiment we record time interval (t) and the corresponding positions (x₁,y₁, x₂,y₂) of electron pairs with specific energies (E₁, E₂). Our coincidence count rates are typically between 0.02 - 2.0 events per second.