There's plenty of advice out there about what to include on your resume: Numbers that prove your achievements, language that appears in the job description (as long as it's relevant to your experience, of course) and impactful verbs like "accomplished" and "improved."

But when it comes to your cover letter, it may seem a bit less straightforward, and sometimes it's hard to know exactly what to include.

"A cover letter is like a written introduction of yourself," says Gorick Ng, a Harvard career advisor and author of "The Unspoken Rules." Ideally, it's about three paragraphs that address who you are, why you are interested in this role and a few relevant experiences that translate directly to it. It's like "a personal sales pitch," he says.

Still, it can be easy to get things wrong. Here's what not to include in your cover letter.   

Addressing it 'to whom it may concern'

To begin with, it is critical to address your cover letter to an actual person instead of writing a generic "to whom it may concern."

If you know who the head of talent acquisition is or whoever is in charge of hiring for the particular team or role, addressing that cover letter to them "shows even just the slightest bit more commitment," says Ng. A lot of people don't submit a cover letter, and if they do, they go that non-specific route. This personal touch will make you stand out.

If you don't know who that hiring manager is, try doing a Google or LinkedIn search to find them, the head of your potential department or anyone else who might be involved with the hiring process.

"You may not get it right but even if you wrote it to someone who's even remotely involved in this process," says Ng, that goes a long way.

Not mentioning the name of the company

Another thing that goes a long way: getting the details of the gig you're applying to right. "An easy way to customize your cover letter that takes literally less than a minute is to mention the company's name and to mention the job title," says Ng.

Include these details in the first paragraph of the cover letter after you've introduced yourself.

This may sound obvious, but many people don't put the time in to write a specific cover letter for each position they're applying for. "What they do is they write a generic block of text that's copied and pasted to everybody," says Ng. Often this doesn't include the company and role they're applying for.

Using language that doesn't reflect the company's culture  

These days, there's a lot you can learn about an employer even before you've stepped foot in their office. And when it comes to your cover letter, you want to give them a sense that you've done your homework and know the culture you're entering.

Peruse the employer's website, blog, any interviews executives might've done with the media and any reviews previous or current employees have written to get a sense of how buttoned up or loose the company is. If none of these help give you a sense of how much flare or personality you can infuse your cover letter with, keep the language neutral and straightforward.

"Really be conscious and present" to the language that you're using, says Christine Sachs, an executive and leadership coach. "Because what's likely going to happen is you have acquired certain ways of speaking, presenting, writing that is a fit for your prior institution, but not necessarily a fit for the new institution."

"I think people try to be clever or they try to add little personal flourishes," says Sachs. If that's not the kind of culture the company has, that may turn them off.

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