The relentless momentum generated by the third movement of Pyotr Tchaikovsky's Pathetique Symphony is the source of one of classical music's most fraught dilemmas. To applaud at the end of the movement or not? The music's frenzy demands applause. Concert convention says not until after the finale. Sunday's much-praised performance at the Edinburgh festival reignited the usual argument. Many applauded after the third movement – some even cheered. Others remained stubbornly silent. The Guardian's line on this issue is predictably liberal – in favour of applauding (or even booing) if it seems right. But the big etiquette talking-point in the Usher Hall on Sunday was not about applause. It was whether it is right – as happened – to admit latecomers after the first movement. On this, liberal instinct falters. Let audiences cheer or catcall as they prefer. But, please, don't allow latecomers in until the interval.