Y is for Year Zero: Grunge killed hair metal. Acid house changed everything. Punk saw off progressive rock. These dividing-line stories are always attractive, always useful for a while— and then always revised. The grandfather of them all, though, has proved harder to shift— the idea that something happened in the early-to-mid-fifties to mark a change of era and fix a boundary of relevance. The next 10 or 20 years, as the 60s slip deeper into unlived collective memory, will be crucial and fascinating (for historians, anyway!).