It depends. In some cases, you can write a general cover letter and use it more than once. However, effective customization leads to more job search success.

What is the purpose of a cover letter?

It informs the employer of the kind of work you want to do, and how you are qualified for a specific position. It also provides that all-important context that a resume cannot achieve. Your resume should never travel alone.

If you look at this definition, you can see that describing how you are qualified for a specific position requires some level of customization on a per-job basis.

The degree of customization might depend on the range of jobs you are applying for. The need for new, unique cover letters increases if each of the jobs you apply for are different in industry, focus or responsibility. One generic cover letter is going to be less effective than letters speaking to the unique needs of each job.

If, however, you are in a situation where you fit into a standardized job function across companies, then you should be able to get away with less customization.

Another factor is job search strategy.

If your job search strategy is to be highly selective, and make contact with only a select group of companies and job opportunities, then unique letters to each of those targets, describing how you fit into their company and their job opportunity is a must.

How committed are you to the job search process?

Cover letter customization is also about your commitment to the job of job searching. If changing jobs is really important to you, then taking the time to really deliver on a compelling cover letter is a necessary and worthwhile investment of your time.

In my experience, one of the most common job search mistakes people make is they apply to far too many jobs in a generic, non-focused way, and then wonder why they don’t get called in for interviews. The opposite strategy is to focus, to understand what you offer and how you fit, and then package that into cover letter, resume, networking and interview language that solidly places you among the viable candidates.

Of course, another factor is the context of the cover letter.

We have been assuming, so far, that the cover letter was in response to a job posting. Eighty percent of jobs, or thereabouts, never get posted. The uninvited letter to a company, and more importantly, a specific person is a smart job search tool, and by definition, must have a degree of customization to it. Likewise, the referral letter requires that you explain the referral and why you are interested in talking to the target contact.

What should change between letters?

What you can do for them will be situation specific. Why you are interested and how you think you fit and are qualified will be situation specific. Your opener might also be situation specific. Use your judgement.

My recommendation is to start by developing a very strong cover letter. The process of doing so once will speed up the writing of subsequent customized cover letters. From there, take the time to customize when it is warranted, based on the importance of your job search, your job search strategy, the kind of roles you are searching for, and the context of the cover letter.