The cover letter follows your CV when you respond to a job ad.
It highlights the strengths of the candidate, the skills, the attitudes and qualities in order to stimulate the reading of the curriculum vitae. The letter, like the CV, is your business card, the first image that a recruiter receives from you.
There is no standard pattern to write a letter of introduction; on the contrary, the letter must be written with the intention of responding to a specific job ad, so customize it every time to add value to your application.
Here are some suggestions and guidelines that might be helpful to write a letter of introduction; it should: