Black to play. Studer–Naiditsch, Biel, July 2020. White intends to meet 1…Bxb5 with 2 Bxe7 Qxe7 3 Qxb5, with an equal position. Which move did Naiditsch play to pinpoint the flaw? Answers should be emailed to chess@spectator.co.uk by Monday 10 August. There is a prize of £20 for the first correct answer out of a hat. Please include your address and allow six weeks for prize delivery.

Last week’s solution 1 Ra8! Kxe7 2 a7! The threat of 3 Rh8 Rxa7 4 Rh7+, winning with a skewer, is decisive. If 2…Kf6 3 Rf8+ and 4 a8=Q.

Last week’s winner W. Boulton, Norwich