When you apply for a job, there is a very good chance your actual qualifications will blend in with those of a sea of other applicants.

In many cases, human resources will present the manager actually doing the hiring with a pile of applicants all at once; in others, he or she will simply open up emailed submissions as they come in. Some of those applicants will be obviously unsuitable for the position, and others may name a salary that's too high, or have other clear issues making it easy to dismiss them.

Many times, though, a sizable number of applicants will make it into the "possible" pile. To guarantee you get into that pile -- even if you're not the perfect candidate for the job -- you need to avoid making one major mistake.

Don't skip writing a cover letter, and don't use a generic one for all job applications. Putting in the extra effort to craft a personalized, customized cover letter for each job you go after can be the difference between success and failure.

A resume is being read

A good cover letter can get your resume to the top of the pile. Image source: Getty Images.

Do cover letters matter?

Admittedly, in some cases, cover letters do not matter. At some companies, screening resumes has been automated; at others, the pre-interview weeding out of applicants is done by lazy HR staff who don't bother reading them.

When that happens, any extra effort you put in will be wasted, but not putting in that effort in the first place is like electing to not buckle your seat belt because you might not be in a crash.

A customized cover letter allows you to introduce yourself to the hiring manager and highlight skills you have that may not be obvious from your resume. More importantly, it's a way to show some personality while explaining how you fit the exact job being advertised.

What should a cover letter do?

You should use your cover letter to point up the ways you fit their position that aren't addressed in your resume. Specifically note any skills you possess that may not be obvious from past positions, schooling, or any certifications you may have that they are seeking. In addition, use this opportunity to address the parts of the ad that are more vague.

For example, if the job posting says "must be a sales leader who works well with other reps," your cover letter can provide some details about your sales experience, and the approach you bring to working as part of a team.

Be brief, but specific. "I'm a sales leader who works well with others," shows you read the ad, but it does not really tell the reader much. "I grew sales by 43% in part by instituting team incentives and holding monthly contests for my reps," would work better.

In addition, a cover letter can address any specific qualifications that the ad asks for that you lack; you it to describe how you'd be able to  either work around the issue, or add the needed skills or certifications to your arsenal.

This is an area to keep carefully within reason. If a job posting seeks someone with CPR certification, it's reasonable to say you don't have it, but could get it within a certain short period of time if you were offered the job. If a position requires an advanced degree and you lack one, well, that's not something you can promise to quickly obtain.

Be yourself, but not too informal

A cover letter allows you to express a bit of personality while subtly selling yourself to the employer. What it is not, however, is a letter to a friend, nor a post on social media: You should keep it formal.

Every cover letter should discuss the specific job and answer questions the person doing the hiring might have about you. It gives you a place to describe your special skills, and connect the dots in a case where you believe you are qualified, but your resume may not make that obvious.

For example, when I was attemping to move from being a news editor and reporter into writing specifically about business and finance, I had no direct experience writing about those areas. In my cover letter, I noted that, in addition to my strong general editorial background, I had spent years running a store, and for a time been in charge of a factory. I made the case that those experiences, coupled with my more traditional news background, made me uniquely qualified for the job I eventually got.

That tactic will not always work, but if I hadn't taken the opportunity to make my case in the cover letter, it would have guaranteed I would not have been considered.

Customizing your cover letter for every job won't get you an interview every time -- but it will lead to more interviews. Sometimes, just the effort might even elevate your application over that of a candidate with better qualifications who did not take the time to do the same.

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