In compiling the chart MRIB employed a sliding scale, meaning that for the lower reaches of the Top 40, airplay counted almost as much as sales. This often meant that the 40-to-20 positions could be very different between the Network and BBC charts. The weight given to airplay diminished the higher one went in the chart, and the Top 10 was meant to be entirely sales-based, although the Network Chart did not register sales from Saturday, the single most important record-buying day, until a week later. It was not unusual for the MRIB and Gallup charts to have different songs at Number One.