Economic expansion during the high middle ages was difficult to analyze through common techniques of data gathering since people of this era did not keep important records. As a result deaths tolls, population size, business transactions, and other life activity went largely un-accounted. One crucial factor contributing to this expansion was population growth. It started slowly in the Carolingian period but had doubled by 1200 A.D. which, by then reached 60 million in Europe. Marriages were greatly encouraged during that time and thriving even though childbirth was the greatest killer of women. The children in turn were living longer lives because living conditions and food of the time promoted greater over-all health. The warm climate made it ideal for agriculture and food was also more abundant. Domestication of animals was mastered and added much needed protein to their diet. Technological advances were another important area of development they helped strengthen agriculture, evolve early forms of transportation and mining, and lead to growth in manufacturing. The population explosion demanded more food to feed the masses so the typical farmland acre of land was producing a crop four times larger than previous eras. Horses were used with more frequency because they could carry loads and cultivate more land than oxen were able to do. Advances in the plow made it easier to cultivate soils without exhausting the ground too rapidly. Nitrogen-fixing crops like beans and peas lessened soil exhaustion. The two-field and three-field system helped preserve valuable soil; this system used two-thirds of land constantly and saved the other part for later use. This crop-rotation helped bring cultivation from 50-67% and allowed for seasonal rotation of crops to combat the harsh weather that was very often problematic. Agricultural specialization was initiated to provide people with crops that were more ideal for a specific location and added to trade. Transportation improvements were needed to supply the produce market that thrived in towns so new Laws were passed to ensure safety on highways, however, those dependent on the road system were left to maintain them. Advancements in seaborne trading along with the replacement of the two-wheeled ox cart—by the four wheeled horse wagon helped towns receive larger quantities and more diversities of food from farther regions. Near surface vein exploitation of ore, iron, tin, and silver made it possible to supply for the increasing demand of tools, weapons, plowshares, construction fittings, and money. Building of religious architecture also increase due to tool advancements and more buildings were erected. The lumber industry was a valuable resource in medieval society helping to expand and connect local economy. Forests were prized and greatly cared for. Florence, Siena, and Pisa were three cities in Italy with the largest populations and were primary centers of important activities. These were primary points of commercial networks which were mainly influenced by Voluntary associations called Guilds. They regulated standards of production, fixed prices, and controlled membership. These associations had a damaging impact on women because they excluded them from equal opportunity. Guilds were also prone to special interest and were filled with plenty of corruption. This era of economic expansion also made though conditions for the poor. Society made attempts to help them but ultimately lost interest because they saw keeping up with the rise in poverty as hopeless. Instead, focus was directed to things like: the just price for bought and sold goods, the issue of lending money, and interests and credit.