Unit 2 covers the three great exchange networks of 1200 to 1450 - the Silk Roads, the Indian Ocean, and the trans-Saharan routes - and asks the same question of each: what made trade grow, and what did that growth change?

Growth had three common causes, and naming them is worth more than listing goods:

• Improved commercial practices 商业惯例的改进 - money economies 货币经济, credit 信贷, and forms of partnership that spread risk.
• Improved transportation technologies 交通技术的改进 - better ships, saddles, and navigation.
• State support 国家支持 - empires that secured routes and taxed the trade benefited from protecting it.

Demand mattered too: luxury goods 奢侈品 from Chinese, Persian, and Indian artisans found expanding markets across Afro-Eurasia, and producers increased output to meet that demand.

The Silk Roads 丝绸之路 carried high-value, low-bulk goods overland across Eurasia: silk, porcelain, spices, and precious metals.

Its enabling innovations were commercial as much as technical - caravanserai 商队旅馆 along the routes, the use of paper money 纸币 and bills of exchange 汇票 in China, and forms of credit that let merchants trade without carrying bullion.

The consequence for producers: Chinese artisans expanded output, and cities along the routes grew wealthy on the traffic passing through.

Empires collapsed and were reconstituted in this period, and the largest transformation came from the Mongol Empire 蒙古帝国.

Mongol expansion facilitated Afro-Eurasian trade and communication by bringing much of the network under a single authority, which lowered the risk of long-distance travel - a period often called the Pax Mongolica 蒙古和平.

Interregional contacts and conflicts also encouraged technological and cultural transfers 技术与文化传播: gunpowder technology moved west, Chinese administrative and medical knowledge circulated, and scholars and craftsmen were relocated across the empire.

The Mongols illustrate the unit's central irony: conquest and destruction produced the conditions for exchange.

The Indian Ocean network moved bulkier goods than the Silk Roads - textiles, timber, grain, and spices - because ships carry more than camels.

Its enabling technologies were maritime: knowledge of the monsoon winds 季风, the lateen sail 三角帆, the dhow 单桅帆船, and navigational tools including the compass 罗盘 and astrolabe 星盘.

Its consequences are the most examinable in the unit:

• The network fostered the growth of states 促进国家成长, such as the Swahili city-states on the East African coast and Srivijaya and Majapahit in Southeast Asia.
• Merchants established diasporic communities 侨居社群 in key ports, introducing their own culture while adopting local ways.
• Long-distance trade depended on and expanded coerced labour and on the movement of people as well as goods.

The trans-Saharan routes crossed the desert on innovations in existing technology - above all the camel saddle 骆驼鞍 and organised caravans 商队.

Gold from West Africa moved north and salt 盐 moved south. The expansion of empires, including Mali 马里帝国, facilitated Afro-Eurasian trade by securing the routes and taxing them - and Mali's wealth and its ruler's famous pilgrimage advertised the region across the Islamic world.

Increased cross-cultural interaction produced diffusion 传播 of literary, artistic, scientific, and religious traditions - the spread of Islam along trade routes, Buddhist and Neo-Confucian influence in East Asia, and the movement of scientific and mathematical knowledge.

An increasing number of travellers 旅行者 moved within and between regions, and their accounts are the period's key evidence.

The fate of cities varied: some declined as routes shifted or invasions came, while others grew into major commercial centres.

The environmental consequences were as large as the cultural ones. There was continued diffusion of crops and pathogens 作物与病原体的传播, and epidemic disease travelled with trade - most catastrophically the Black Death 黑死病, which moved along the same routes that carried goods and killed a large share of the population of Afro-Eurasia.

The unit's reasoning skill is comparison. Compare the networks on what they carried, what made them possible, and what they changed - not on which was "most important".

Worked example (a real AP exam question). AP World short-answer questions frequently ask students to "Explain one similarity" or "Explain one difference" between two trade networks in this period. A full-mark answer names the feature and applies it to both, then explains: "Both the Silk Roads and the Indian Ocean network expanded because of improved commercial practices, but the Indian Ocean carried heavier, lower-value goods such as timber and grain because ships could transport far greater bulk than overland caravans." Notice the shape: the because clause is what turns a difference into an explanation, and without it the answer only describes.

• Explain growth with the three causes - commercial practices, transport technology, state support - rather than listing trade goods.
• Match cargo to carrier: overland routes move high-value, low-bulk goods; maritime routes move bulk.
• Learn the enabling technologies by network: caravanserai and credit; monsoons, lateen sail and dhow; the camel saddle.
• The Mongols' examinable role is facilitating exchange, not conquest for its own sake.
• The Black Death is the environmental consequence to name, and it travelled on the trade network itself.
• On difference questions, always add the because. A stated difference without a cause is only half an answer.