LONDON - The surge in affection for Kate Bush’s Running Up That Hill looks likely to send her to number one this week. The 80s ballad is already at number two, thanks to its use in the hit Netflix series Stranger Things. Its chances of making the top have now increased, after the Official Charts exempted it from a rule that penalises older songs when they are streamed.

Running Up That Hill is currently the UK’s most-streamed song, averaging 700,000 plays per day on Spotify. In previous weeks, those streams would have counted as 3,500 “sales”. This week, that figure will double to 7,000. That’s all because of an esoteric, but important, rule dictating the way the charts are calculated in the streaming era.

The Top 40 used to consist of songs you could buy in a record shop, and every sale was equal. If a record label hadn’t pressed enough copies of a 7-inch or CD single, it might miss the chart altogether.

In some instances, record labels even stopped manufacturing a hit single in order to convince people to buy the album instead (Wet Wet Wet’s Love Is All Around and the Fugees’ Killing Me Softly both suffered this fate). But in the streaming era, every song is available everywhere all the time. This poses the charts with a problem. If you counted streams on a like-for-like basis, classic songs like The Killers’ Mr Brightside would never leave the Top 40; and a modern hit like Ed Sheeran’s Bad Habits would currently be spending its 50th week in the Top 10. In order to give other artists a chance and keep the charts from becoming stale, the record industry invented something called “accelerated decline”.