A pledgor and a pledgee shall conclude a pledge contract in writing.

第六十四条出质人和质权人应当以书面形式订立质押合同。

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A pledge contract could be a separately concluded contract or a pledge clause of the master contract.

质押合同可以是单独订立的合同，也可以是主合同中的担保条款。

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Article 210 To create the interest of a pledge, the parties concerned shall conclude a pledge contract in written form.

第二百一十条设立质权，当事人应当采取书面形式订立质权合同。

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The site says the pledge is a "moral commitment" not a "legal contract".

BBC: US billionaires pledge 50% of their wealth to charity

The pledge is a moral commitment to give, not a legal contract, and it does not involve pooling money or supporting a particular set of causes or organizations.

FORBES: Ten More Tycoons Sign Buffett And Gates Giving Pledge: Dalio, Kinder, DeJoria