写信的原则（Writing Principles）已从原来的3（Conciseness, Clearness, Courtesy）发展到目前的7个"Ｃ"：

Completeness, Clearness, Concreteness, Conciseness, Correctness, Courtesy,Consideration)

实例

Dear Sirs,

　　With reference to your letter of April 9, we are pleased to accept your offer of 100 tons of Copper Wire as per your Offer Sheet No.8/070/02B. Please go ahead and apply for your Export License. As soon as we are informed of the number of the Export License we will open the L/C by cable.

Copper Wire

大意：
关于你们四月九日函，我们高兴地接受你们第8/070/02B号报盘单所报100吨紫色铜丝。请着手办理申请出口许可证。　一经接到出口许可证号码的通知，当即电开信用证。

要求书信的"完整"，　理由有三：
１．一封完整的书信比一封不完整的书信，有更大的可能性带来预期的效果；

２．一封完整的书信，有助于建立和表达友善关系；

３．一封完整的书信，可以避免由于遗漏重要情况（情报）所导致的诉讼（Lawsuit）；

有时，某些不显眼的书信或文件，由于所提供的情况完整而又生动有力（Complete and Effective）而成为极为重要的文件。一封信写得是否完整，建议用五个"Ｗ"来检验，即：Who, What, Where, When 及Why(包括How)。例如:在定货的信中，必须明确说明"需要什么商品(What you want)、何时需要(When you need the goods) 、货物发到何地何人收(to Whom and Where the goods to be sent) 、如何付款(How payment will be made) 如对对方的要求作出否定的答复时（如不能报盘，不能理赔等）应说明理由，为什么(Why)

Clearness(清楚)

（一）．避免使用可能产生不同理解或意义不明确的词汇〈例一〉As to the steamers sailing from Hongkong to San Francisco, we have bimonthly direct services. (从香港到旧金山有直达船，)但是bimonthly 究竟是一个月两次即半月一次呢，还是两个月一次？不明确。因此，最好清楚明白地说明"一个月两次"还是"两个月一次"：

（a）We have two direct sailings every month from Hongkong to San Francisco.（每月两次直达船）（b）We have semimonthly direct sailing from Hongkong to San Francisco.（每半月一次直达船）（c）We have a direct sailing from Hongkong to San Francisco every two month.（每两个月一次直达船）

（二）注意修饰词的位置，有时修饰词的位置不同，会导致不同的含意如：a) Please let us know what you wish us to do about this matter as soon as possible; b) Please let us know as soon as possible what you wish us to do about this matter. 以上两句中的""修饰的内容不同：a) 你们要求我们尽快做些什么。b) 请尽快告诉我们。所以a) 可译成"请告你们要我们为此尽快做些什么"；  b) 可译为"请尽快告知，你们要我们为此事做些什么"。

（三）注意代名词，关系代词和先行词的关系按照英语习惯，句子中的代名词和关系代词一般是指离得最近的名词，因此要特别注意，以免引起误解或不清楚。例句：不好：They informed Messrs.John & Smith that they would receive an answer in a few days. 较好：They informed Messrs. John & Smith that latter would receive an answer in a few days.

（四）注意前后一致和紧凑，连贯不一致：Being a certified public accountant, I am sure you can help us. 一致：Being a certified public accountant, you can surely help us. or; As you are a certified public accountant, I am sure you can help us.