501(c)(3) vs (c)(4) vs (c)(6): what the letter after the number means

“Nonprofit” is not one thing. The IRS recognizes more than two dozen kinds of tax-exempt organization under section 501(c) of the tax code, and the number in the parentheses changes what the organization is allowed to do — whether your donation is tax-deductible, whether it can lobby Congress, whether it can back a candidate. Three of those types cover almost everything you’ll actually encounter, and the differences between them explain a lot of otherwise-confusing things about how advocacy and money work in the US.

This is a companion to the donor’s guide to reading a 990 — think of it as the front door. Before you evaluate an organization’s finances, it helps to know what kind of organization you’re looking at, because the rules it plays by depend entirely on that letter-and-number.

The charity: 501(c)(3)

This is what most people mean by “nonprofit.” A 501(c)(3) exists for charitable, religious, educational, or scientific purposes — your food bank, your local museum, your university, your church, the Red Cross.

Two things define it:

• Your donation is tax-deductible. This is the big one, and it’s nearly unique to (c)(3). If you itemize, giving to a (c)(3) reduces your taxable income. Giving to the other types on this list generally does not.
• It can’t do politics. A (c)(3) is barred from campaign intervention entirely — it cannot endorse, oppose, or fund candidates for office (this is the “Johnson Amendment”). It can lobby on issues, but only in a limited way; lobbying can’t be a substantial part of what it does.

That deductibility comes with strings, and the no-politics rule is the price of it. Worth knowing: (c)(3)s split into public charities (funded by many donors — most of what you interact with) and private foundations (usually funded by one family or company, and filing a different form, the 990-PF). Same deductibility, different rules and different filing.

The advocacy group: 501(c)(4)

A 501(c)(4) is a “social welfare” organization, and the easiest way to understand it is as the mirror image of a (c)(3) on exactly the two points above:

• Your donation is not tax-deductible. You give with after-tax dollars.
• It can do politics. A (c)(4) can lobby as much as it wants, and it can engage in political campaign activity — as long as that’s not its primary purpose.

That trade is the whole point. A (c)(4) gives up the donation deduction in exchange for the freedom to advocate. It’s why so many well-known groups exist as a (c)(3)/(c)(4) pair: a charitable arm that takes deductible donations for education and litigation, and an affiliated advocacy arm that lobbies and campaigns. The Sierra Club and the Sierra Club Foundation, the ACLU and the ACLU Foundation — same brand, two entities, two sets of rules. It’s also why “(c)(4)” shows up in every conversation about dark money: because these groups can spend on politics and generally don’t have to disclose their donors.

The trade association: 501(c)(6)

A 501(c)(6) is a business league — a chamber of commerce, a trade association, a professional society. Think the local chamber, the National Association of Realtors, a bar association. It exists to advance the shared interests of its members, not the general public.

• Dues aren’t a charitable deduction, though a business may be able to deduct them as an ordinary business expense (minus any portion that goes to lobbying).
• It can lobby freely on behalf of its industry, much like a (c)(4).

If a (c)(3) serves the public and a (c)(4) advances a cause, a (c)(6) advances a business sector.

The one table worth keeping

*Public charities report large donors to the IRS on Schedule B, but that schedule is redacted from the public copy.

Why a donor should care

Two practical reasons. First, deductibility: if the tax break matters to you, confirm you’re giving to a (c)(3) — the others won’t get you the charitable deduction. Second, what your money actually does: giving to the (c)(3) arm of an organization funds programs, education, and litigation; giving to its (c)(4) arm funds advocacy and political muscle. Same cause, very different levers, and the letter tells you which one you’re pulling.

And here’s the useful part: all three file a public Form 990, so once you know the type, the reading skills from the companion post carry right over — with the caveat that a (c)(4)’s or (c)(6)’s “program” spending includes advocacy, and private foundations file the different 990-PF. Know the letter first; then read the numbers.

This is a plain-language overview, not tax or legal advice. The authoritative source is IRS Publication 557 and the IRS exempt-organization pages; an organization’s exact status is on its 990 or in the IRS Tax Exempt Organization Search.