A semi-block style letter is a less formal version of a block or a full-block letter with the differences being the sender's address, date, reference or attention line and complimentary closing. In addition, the signature lines are located direct center or slightly right of center, along with indented paragraphs. The semi-block style is also called modified semi-block because it is a modified version of the block or full block style.
Business letters generally contain the same elements, and these elements are expected in business correspondence. The semi-block style of business letter, with indented paragraphs and off center elements, appears less formal than the block and full block styles, where all elements of the letter are left margin aligned, and there are no indentations. The block and full block styles display a uniformity with the margin alignment, which gives a business letter a more formal appearance. Depending on the purpose and content of the letter, a semi-block format may be more desirable for less formal business correspondence, such as thank you messages or announcements of events. Letters to, and dealing with, smaller businesses with less corporate oversight and formality should use the semi-block style to create a more personalized correspondence than a standard corporate business letter.