Legal analysis: 1. The recommendation letter is witnessed by a lawyer. As long as there is a written certificate signed by both parties, both parties can sign it voluntarily. It can be used as an effective legal basis. If another lawyer witnesses, it will be as efficient as legal documents. 2. The letter of guarantee in a couple's marriage is usually that during their life, one of them has committed wrong acts, such as extramarital affairs, drug abuse, gambling, domestic violence, etc. Because the behavior was exposed or persuaded by relatives and friends. In order to save his marriage, he admitted his mistake and made various written promises such as "committing another voluntary divorce", "returning the property to the other party", "giving up the custody of the children" and "giving the other party financial compensation". For this kind of guarantee, because of its different contents, the court finds different situations. To sum up, the general guarantee requires the voluntary signature of both parties or the witness of relevant lawyers, and this guarantee can have legal effect.

Legal basis: Article 110 of the Civil Procedure Law of People's Republic of China (PRC). When the people's court deems it necessary, it may require the parties to appear in court in person and be questioned about the relevant facts of the case. Before questioning the parties, they may need to sign a letter of guarantee. The letter of guarantee shall contain true statements, willingness to accept punishment if there are false statements, etc. The parties shall sign or seal the letter of guarantee. If the party who bears the burden of proof refuses to appear in court, refuse to be questioned or refuse to sign a letter of guarantee, and the facts to be proved lack other evidence, the people's court will not recognize the facts it claims.