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。本系列文章从这7个"C"，为您讲解商务信函的书写规则。
〈例〉 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）
〈例〉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：
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"。
〈例〉 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.
〈例〉 连贯不一致：Being a certified public accountant, I am sure you can help us.
连贯一致： Being a certified public accountant, you can surely help us. / As you are a certified public accountant, I am sure you can help us.
商业书信要写得使人一读就明白，因此必须按照内容仔细和恰当的分段落，一般的分段落原则是一个段落一个意思（A paragraph for each point is a good general rule.）。
It is reported in a domestic newspaper that the Iranian Central Bank has instructed the commercial banks to suspend their business of opening a new letter of credit as from the 3rd May for financial reason of foreign currency. Although it is said that this arrangement would be a temporary one and with establishment of new import policy this emergency arrangement would be lifted, we are much concerned about the outcome of this movement toward restriction of import to Iran and shall be obliged if you will kindly keep us well advised of development of this new arrangement especially in connection with import from France.
以上这封信只有一段，供两句。第一句39字，第二句66句。 读后虽然能基本了解其内容， 但读起来很舒服。信的内容大体有三点： 1. 伊朗中央银行因外汇原因，通知个商业银行从五月三日起停止开立信用证； 2. 这临时措施，新的进口政策一订，这个措施可能取消； 3. 希望对方随时告之有关情况。按照这三点，适当地分分段落，把信重新改写一下，可能读起来比原来的清楚易懂：
It is reported in a vernacular newspaper that the Iranian Central Bank has instructed the commercial banks to suspend their issuing new Letters of Credit as from the 3rd May because of its decreasing foreign currency funds. Furthermore it is said that the suspension seems a temporary one and will be released with establishment if a new import policy.
We are much concerned, however, about the outcome of this movement toward restriction of import to Iran. We, therefore, shall be obliged if you will keep us well informed of development of this new state of affairs, especially in connection with imports from France.