Manufacturers depend on commercial services to provide them with raw materials and components and to distribute their finished goods. Without manufacturers, on the other hand, commercial workers would have nothing to do.
Secondary production is dependent on primary production for the provision of raw materials. Tertiary production is dependent on secondary production for the supply of manufactured goods.
Eg: Construction industry makes use of raw materials such as sand, water. They also use manufactured good like cement and many commercial services like advertising, transport and financing. Commercial services like these enable the primary and secondary industries to function. So they are inter-dependent.
Let us look at an example.
Irafan completed secondary school in the year 2010. However, he did not go well in GCE O’Levels and had very little hope of getting a job. Therefore he decided to earn some income by repairing washing machines and doing other mechanical work. This is a skill he picked up while he used to visit his friend’s garage the previous year. To his surprise he found that his services are very much in demand. As soon has he put a little classified ad in a ad-listing website, he started getting calls from home which needed repairs to washing machines and other maintenance work. He soon took a motor-cycle for hire-purchase from Borother’s motor company.
He also opened a bank account to keep his money. He soon opened his own garage and now has three employees who go out and do the work. To open the garage he took a loan from the bank. To manage the contracts with his employees, he sought the help of a lawyer in-order to do it properly.
Here Irfan is involved in direct services. We can see that he is using aids to trade in his business.