How to Format a Business Letter (Block, Modified Block, and Semi-Block)

There are three formats accepted as standard for a printed business letter, and choosing between them is less about correctness than about signal. The block format — every line flush against the left margin, no indents, an open spacing between paragraphs — is the modern default in most American corporate correspondence. It looks clean, scans quickly, and prints predictably across email-to-PDF workflows. If you are writing a letter for the first time and unsure what to use, default to block. It will not be wrong in any common context, and recipients have come to expect it.

The modified block format softens the look of a block letter by aligning the date, sign-off, and signature with the right half of the page. It reads as slightly more traditional, and it remains common in legal, banking, and academic contexts where a more formal visual register suits the relationship. Modified block is also useful when a letter is mailed in a window envelope, because it keeps the recipient's address comfortably aligned with the rest of the page. For a more granular breakdown of the conventions discussed here, see the companion professional drafting reference we maintain alongside this guide.

The semi-block format adds a paragraph indent — usually a half-inch tab — to the start of each new paragraph. It is the most traditional of the three formats, the one closest to printed correspondence from before computers, and it carries a slight literary feel. Use it when the letter is personal in tone but still formal in purpose: a letter of condolence to a long-time client, a thank-you to a mentor, a hand-signed letter from a small firm. Semi-block looks out of place in modern corporate email but right at home on letterhead with a wet signature. Practitioners who write in regulated industries may also find our compliance-aware editorial checklist a useful next step.

Whichever format you choose, keep the conventions of the format consistent through the entire letter. Mixing indented paragraphs with a left-aligned sign-off, or block paragraphs with a centered date, makes the letter look like a bad merge from two templates. The rule is simple: pick a format, follow its rules through to the signature, and the visual coherence of the page will quietly do half the work of seeming professional.