Since the 15th century, people living north of the Alps have been using rennet – a substance from the stomach of a cow – to make hard cheeses. This hard cheese was greatly appreciated because it was much more durable than the cottage cheese that people had been making before. It allowed them to create a milk supply for the winter. And so were the foundations of the Swiss cheese culture laid… Until the 18th century, hard cheese was made only in the summer in the Alps. The milk yields of those times meant that the cows were dry for a long time in the winter and the rest of the milk produced was used the household. This changed in the early 19th century, when the practice of intensive agriculture enabled cheese to be produced in the valleys and during the winter – i.e. at times when the cows were being fed hay. The cheese made year-round in the valleys in the mountainous regions is now known as mountain cheese – to distinguish it from Alp cheese.