Production leveling — smoothing the volume and mix of production over time to eliminate the peaks and valleys that create overburden (muri) and unevenness (mura). Instead of producing large batches of one model followed by large batches of another, heijunka distributes production of all models evenly throughout the day. This requires the fast changeovers that smed enables and the flexible workforce that standard-work trains. Heijunka is the least intuitive TPS concept for mass-production thinkers — it appears to sacrifice economies of scale. But taiichi-ohno argued that the apparent efficiency of large batches is illusory: they create inventory (waste), amplify demand variation upstream (the bullwhip effect), and hide quality problems. Leveled production creates stable demand on every upstream process, making just-in-time and kanban possible. The heijunka box (a visual scheduling device) makes the production plan visible and is one of TPS's signature visual management tools. Heijunka was systematized during the tps-maturation-1965-1980 era.