加拿大多伦多新飞扬留学
一般而言，美国大学入学要求学生提供两封推荐信，分别来自于两位高中的任课老师。而任何一般的规则都会有例外，大学入学政策也是这样的。所以当你提交推荐信时，必须检查每个大学的具体政策。
关于这两封必须的推荐信，大学更希望它们来自于核心课程的老师，如数学，英语，科学，社会科学（历史，经济学等）和除了中文以外的外语。如果可能，避免选修课的老师，如音乐，艺术，体育等。
虽然不是一项硬性规定，大学通常喜欢来自于不同主题课程的老师，例如生物学和历史，而不是生物学和数学。当然，前题是这两个课程的老师都会写出同样强有力的推荐信。如果在上面的例子中，你与历史老师的关系不怎么样，但与数学老师的关系很好，那就请你的数学老师写。底线是，提交两封最好的推荐信，无论是什么课程的老师。
很难评估推荐信在录取过程中的重要性，它只是在录取排名中，十几个甚至几十个被招生官评估的项目之一。应该强调的是，一般来说，入学申请中最重要的部分是高中成绩单(GPA)，考试成绩（SAT，ACT， TOEFL），课外活动（活动，人才，领导，社区服务等）的总和，作文和面试。我会把推荐信的重要性排在那些项目的后面，其主要原因是，几乎所有的推荐信都是积极的，对于不同的学生来说都是相似的。只有当推荐信写得非常积极或非常负面，才会有更大的效果。当它们在那些极端的情况时，推荐信才有更大的份量。而消极的推荐信会严重损害被录取的机会。
除非是特殊情况（例如，学生被要求在提交之前翻译推荐信），推荐信是应该保密的。
除了标准的两个教师的推荐信之外，许多大学允许一个或两个补充推荐信。写这些推荐信的人来自于学生所在学校之外、学生为其做了令人印象深刻的工作。例如：他们可能来自某个实习的主管，学生做过研究的大学教授，或是老年人中心的主任，学生在那里做了许多小时的志愿服务。这些类型的推荐信可以与教师的推荐信一样重要（或在某些情况下，甚至更重要）。如果一个学生与这样的某个人有良好的关系，并且取得了引人瞩目的成就（而且所申请的大学允许补充推荐信），那是绝对应该提供的。
在通用申请程序中，大学对他们的招生政策和补充推荐信是有特别的说明的。通用申请程序有一个条目，专门解释推荐人提交其推荐信的过程。
英文原文：
The Role of Recommendations in the Admission Process to US Colleges/Universities
By Don Betterton
Although there are exceptions to any general rule about college admission policies (one has to check each individual college for its policies), the typical recommendation requirement is for two teacher recs.
Of the two recommendations, colleges prefer they come from teachers in the core subjects (math, English, science, social science (history,economics, etc), and a foreign language other than Chinese. If possible, avoid recs from teachers in elective subjects like music, art, physical ed, etc.
Although not a hard rule, colleges generally prefer to see recs from teachers in different subject areas, for example, Biology and History, rather than Biology and Math. However, this assumes that both teachers would write an equally strong rec. If, in the example above, you don't have a good relationship with the History teacher and do have a good relationship with the Math teacher, ask your Math teacher to write. The bottom line is to submit the two best recs regardless of subject.
It is hard to put a weight on recs in the admission process. They are one of dozens and dozens of items an admission reader will look at in coming up with an admission rating. Once again, generally speaking, the most important parts of an admission app are the high school transcript, test scores, the sum of everything the student does outside of class (activities, talent, leadership, community service, etc), essays, and the interview. I would place recs next, mainly because nearly all of them are positive and are rather similar in writing about a student's attributes. Where recs can have a larger effect is when they are extremely positive or very negative. Recs will carry more weight when they are at the extremes. A negative rec can severely damage admission chances.
Unless it is an unusual circumstance (for example, the student is asked to translate the rec before it is submitted), recommendation letters should be confidential.
In addition to the usual standard of two teacher recs, many colleges allow for one or two supplemental recs. These recs would come from someone outsidethe school with whom the student has done impressive work. Examples: they might come from a supervisor in an internship, a college professor for whom the student did research, or the director of an old age home where the student put in many hours of volunteer service. These types of recs can be as important (or in some case, even more important) than the teacher recs. If a student has a good relationship with such a person as well as having made a noteworthy accomplishment (and the college allows for a supplemental rec), definitely submit one.
In the Common App, colleges specify their policy on school and supplemental recs. The CA also has a section that explains the procedure for recommenders to submit their recs.
