Collection letters are usually sent in a series.The first tends to be mildest and most understanding,with the letter getting gradually more insistent.The final letter in a series, when all else has failed, threatens to turn the matter over to a lawyer or collection agency. Of course,the tone of any letter in the series will vary,from positive and mild to negative and strong, depending upon the past pay ment record of the particular customer. The intervals between the letters may also vary,from ten days to month at the start,from one to two week later on.