A delivery address line is six fields, written in one fixed order

The primary number is the house, building or box number assigned to the property by the local addressing authority. It leads the line, it carries no punctuation, and it is the one field you may never abbreviate, expand or reformat.

Two habits break it. The first is writing a fraction or a hyphenated number in a form that the receiving system splits at the wrong character — 1129-B often arrives parsed as house 1129 with a stray token. The second is padding with zeros to make a column line up: 01129 and 1129 are not the same key in most address databases.

If a building genuinely has a lettered or fractional primary number, keep it attached with no space and let the secondary designator in field five carry the unit instead. That keeps the parse unambiguous.

A directional that sits before the street name is a predirectional; one that sits after it is a postdirectional. Same word, different field, different meaning — and a great many towns have both a North Ridge Road and a Ridge Road North.

Directionals are written as one or two letters with no periods: N, S, E, W, NE, NW, SE, SW. The trap is that a directional word can also be part of the street name itself. In “West Ridge Road” the W may belong to field two; in “Westridge Road” there is no field two at all, because West is welded into the name. A single space decides which of two different streets you are addressing.

The street name is written in full. It is the field where a well-meaning cleanup script does the most damage, because the words inside a street name look abbreviatable and are not.

A street named Saint Andrews does not become ST ANDREWS — ST is the published abbreviation for STREET, so that edit silently changes the suffix field. Fort, Mount and Point behave the same way. Numbered streets keep their ordinal ending: 7TH, not 7 and not SEVENTH.

Publication 28 asks for the whole line in uppercase without punctuation for automated mail. That is a formatting rule for the printed piece, not permission to shorten the words inside the name.

The suffix says what kind of thoroughfare it is, and the Postal Service publishes exactly one standard abbreviation for each one. Several common spellings map onto the same standard form, which is why “Blvd”, “Boul” and “Boulevard” all reduce to BLVD.

Two entries in the published list catch people out every time. Way abbreviates to WAY, but the plural Ways stays WAYS. Park and Parks both reduce to PARK, so the plural disappears entirely — if a system round-trips the expansion it will hand back the singular, and that is correct, not a bug.

The full table of primary suffix names, commonly used forms and standard abbreviations is Appendix C1 of Publication 28, linked at the foot of this page.

Apartment, suite, floor, room, unit: this is the field that says which part of the building, and it belongs on the same line as the street address whenever it fits. Pushing it up to its own line above the street is the single most common cause of an address that validates as a building but delivers to nobody.

The right-hand column is the part people miss. BSMT, FRNT, LBBY, REAR, SIDE, UPPR, OFC and PH describe a location rather than a numbered unit, so no number follows them. The others are incomplete on their own: APT alone is not an address, APT 4B is.

When you have a designator that appears on no published list, the pound sign is the documented escape hatch — 1129 W RIDGE RD # 200 — and it takes a space before the number.

City, two-letter state, ZIP. It looks like the easy one and it carries a rule that surprises people: the city name that belongs here is the one the City State file recognises for that ZIP code, which is not always the town you would say out loud.

Plenty of addresses sit inside one municipality's boundary while their mail is sorted under a neighbouring city's name. Both are true statements about the property; only one of them is the last line. If validation keeps rewriting your city, it is not being pedantic — it is returning the recognised name for that ZIP.

The ZIP+4 add-on is written with a hyphen and no space, and it is worth carrying because it is what resolves the delivery point down to a specific unit rather than a whole building. Publication 28 defines a complete address as one that matches the current ZIP+4 and City State files finely enough to obtain that delivery point code.

• Does every row split cleanly into the six fields, with nothing left over?
• Are directionals stored separately from the street name, rather than glued to it?
• Does any street name contain ST, DR, MT or FT as a word that a cleanup pass could mistake for a suffix?
• Do numbered streets keep their ordinal ending?
• Is the secondary designator on the street line rather than a line of its own?
• Does every APT, STE, BLDG, FL, RM or UNIT have a number after it?
• Does any BSMT, FRNT, LBBY, REAR, SIDE, UPPR, OFC or PH have a number after it that it should not?
• Do the city and ZIP agree with the City State file, rather than with local habit?