Where a phone number is really from, and why.
Area codes, ZIP codes, counties, and international dialing formats -- sourced from NANPA, USPS/Census data, and current carrier records, with the split-and-overlay history and local context a one-line answer leaves out.
Area Codes
Browse allState and province lists, individual lookups, and how the numbering plan actually works.
Every Texas Area Code, and the Overlay Stack Behind Each Metro
Texas has 29 area codes today. Dallas alone accounts for four of them stacked on top of each other -- here's the full list, plus which codes overlay which in Houston, San Antonio, Austin, and Fort Worth.
Every California Area Code, and the Family Tree Behind Los Angeles's Six
California has 41 area codes today, more than any other state -- and Los Angeles alone traces back through six generations of splits and overlays to a single 1947 code, 213.
How US Area Codes Actually Work
A US phone number has three parts, not one. Here is what each part actually identifies, why New York got 212 specifically, and the real difference between a "split" and an "overlay."
ZIP Codes
Browse allCity ZIP code lists and county breakdowns, sourced from USPS/Census data.
Atlanta ZIP Codes: The Full List, and Why Some Say Fulton and Others Say DeKalb
Every ZIP code USPS assigns to Atlanta, Georgia -- which of the two counties each one actually sits in, why several Atlanta suburbs mail-address as their own separate cities, and the one Atlanta ZIP that belongs to the IRS.
New York City ZIP Codes: All 311, by Borough
NYC's five boroughs use 311 ZIP codes between them -- and Queens is the one borough where the ZIP code map doesn't run through neighborhood names most outsiders have never heard of, not the borough name itself.
Texas ZIP Codes: How the State's 2,600 Codes Are Organized
Texas doesn't have one ZIP code system, it has a range spanning 73301 to 88595 -- and which major city has the most ZIP codes isn't the one with the most people.
Counties
Browse allWhich county a major US city actually sits in.
What County Is Chicago In?
Chicago is entirely inside Cook County -- all 84 of its ZIP codes, no split with a neighboring county. Cook is the second-most-populous county in the US, and it's losing that spot to Houston's Harris County.
What County Is Houston In?
Houston is entirely inside Harris County, its address labeled that way for every one of the city's 178 ZIP codes -- and Harris County is about to overtake Chicago's Cook County as the second most populous county in the US.
International Dialing
Browse allCurrent dialing formats for calling abroad from the US.