/sources

Citation manifest

Every claim on this site is anchored to one of the primary IRS sources below. Each links straight to the official document so you can open it and confirm the claim yourself.

Source	Scope
IRS Pub 1635 — Understanding Your EIN https://www.irs.gov/pub/irs-pdf/p1635.pdf	Primary reference for codes 101–115 and SS-4 reference text
Form SS-4 Instructions https://www.irs.gov/instructions/iss4	Where to file, fax/mail addresses, RP definition
Where to File Form SS-4 https://www.irs.gov/filing/where-to-file-your-taxes-for-form-ss-4	Current IRS fax numbers and mailing addresses by RP location
26 CFR §301.6109-1 https://www.ecfr.gov/current/title-26/chapter-I/subchapter-F/part-301/section-301.6109-1	Federal regulations defining taxpayer ID requirements
IRS Business & Specialty Tax Line https://www.irs.gov/businesses/small-businesses-self-employed/telephone-assistance-contacts-for-business-customers	800-829-4933 — escalation phone path for codes 109, 111, 112
IRS Form 147C https://www.irs.gov/businesses/small-businesses-self-employed/lost-or-misplaced-your-ein	EIN verification letter — used for codes 111, 112
IRS Online EIN Assistant — operating hours https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-employer-identification-number-ein-online	Mon–Fri 7am–10pm ET — context for code 110

These are the primary sources behind every claim on the site. Open any of them to confirm the wording directly. Informational, not tax advice.