(1) Supplementing the items omitted in the original agreement. For example, after signing the house lease contract, it was found that there was no agreement on rent-free period and delivery time. These issues can be re-signed supplementary agreements. (2) The original agreement is unclear and needs to be supplemented. For example, as stipulated in the loan agreement, the borrower pays the lender interest with an annual interest rate not higher than 24%. This agreement is not clear, the annual interest rate 10% is not higher than 24%, and the annual interest rate 1% is not higher than 24%. This makes it impossible to determine how to pay interest. At this time, a supplementary agreement can be signed to clarify the interest rate. (3) Modifying or changing the original agreement. For example, after the custom-made furniture contract was signed, the customer found that the furniture to be customized did not match the decoration style of his home, but the contract was signed. At this time, as long as both parties agree, they can also make supplementary agreements on re-selecting the style of furniture. A supplementary agreement is an agreement signed by the parties under the condition that the contents of the contract are not agreed or the agreement is not clear. After signing the contract, if it is found that the contents need to be supplemented, a supplementary agreement can be drawn up. According to Article 5 10 of the General Principles of the Civil Law, after the contract comes into effect, the parties have no agreement or unclear agreement on quality, price or remuneration, place of performance, etc. They can supplement the agreement; If a supplementary agreement cannot be reached, it shall be determined in accordance with the relevant provisions of the contract or trading habits. After the contract comes into effect, the parties have not agreed or clearly agreed on the quality, price or remuneration, place of performance, etc. They can supplement it by agreement. Accordingly, only after the main contract comes into effect, if it is difficult to perform because of the lack of agreement or unclear agreement on quality and price, can the "agreement" supplement the matters that are not agreed or unclear, so as to facilitate the performance of the main contract and realize the purpose of the parties to conclude the main contract. There is a master-slave relationship between the existing contract and the supplementary agreement. The existing contract is the master contract and the Supplementary Agreement is the slave contract. As the main contract, the existing contract does not depend on the existence of supplementary agreements, or exists independently without the restrictions of supplementary agreements. On the contrary, the "supplementary agreement" must be based on the existence of the existing contract and cannot exist independently.