An excuse letter is usually written to an employer, school or college explaining the reason that why you were not able to attend work or school for a period of time. The reason for your leave of absence could be anything like you had been sick, you attended a wedding event, and you attended a funeral and more such reasons. Sometimes you will have to inform your leave of absence in advance with an excuse letter or provide the same later when you return to work. An excuse letter to work should carry a formal tone throughout. There are some suggestions at f-origin site on the types of excuses you can give at your workplace.
When writing an excuse letter to your employer, you have to understand the leave policies of your company beforehand. Get to understand if your boss would need the excuse letter in written format or through email or just a phone call would do. Also get to know how many days in advance you need to inform your boss about your leave of absence.
When writing the letter you can keep it brief. Do not get into detail like explaining the symptoms of illness or the personal problem. You need to mention the dates and days you will be absent and explain the reason in short. You can add a few words about your interest in helping your job so that it does not stagnate. You can inform that you will be available online for any kind of assistance like replying through email.
The letter should have from address, date, to address, the body explaining your leave of absence and an ending note with your signature. If possible try to send the excuse letter in advance before you miss work. This will be helpful for your employer to assign the work to someone else if you are going on a long leave.