Employers or HR professionals will most likely first read your covering letter, and only then decide whether they will read your CV and the rest of your application. So the main objective of the covering letter is to hook.
You need to show that you are truly convinced about being the right person for the job, about having the right competences. If they can feel that you are convinced, they will start getting convinced too, and they will want to know more – for which reason, you are sending them your CV and suggesting a talk, aka interview.
The covering letter should be of 1 page maximum, a bit shorter is even better.
You should not write it as a brief version of the CV, but highlight your strong motivation for performing the job.
Crosscheck any potential misspellings, and give it nice graphics – print it before sending it out to see exactly the same layout that they will see. Already - at the first impression - your letter needs to look brief, clear and professional.
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