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Reference explainers for the SEC filing forms developers, traders, and quants actually deal with. Every page is written for working professionals: no jargon, no fluff, with the API view of the data at the bottom.

• What is a Form 4 filing? (SEC insider trading disclosure)A Form 4 is the SEC filing that company insiders submit within two business days of trading their own company's stock. Here's what it contains and how to read it.
• Form 4 transaction codes (P, S, A, M, F and the rest) explainedEvery transaction code that can appear on an SEC Form 4 filing, what it means, and how seriously to read each one as a signal.
• What is a 13F filing? (Institutional holdings disclosure)A 13F is the quarterly SEC filing where institutional investment managers with over $100M in assets disclose their long equity positions. Here's what it contains, when it's filed, and how to use it.
• What is an 8-K filing? (SEC material events disclosure)An 8-K is the SEC filing that public companies use to disclose material events between quarterly reports — earnings, executive changes, acquisitions, and more. Here's what it covers and when it's filed.
• What is a 10-K filing? (SEC annual report)A 10-K is the comprehensive annual report public companies file with the SEC. Here's what it contains, when it's filed, and how it differs from a 10-Q or an annual report to shareholders.
• Complete list of SEC filing types (and what each one means)Every common SEC filing type — 10-K, 10-Q, 8-K, S-1, Form 3/4/5, 13F, 13D/G, DEF 14A, Schedule 14A, and the rest — explained in plain English with links to deeper guides.
• What is a Form 3 SEC filing? (Initial beneficial ownership statement)Form 3 is the SEC filing that new insiders submit to establish their starting ownership position. Here's who files it, when, and what it contains.
• What is a Form 5 SEC filing? (Annual insider ownership report)Form 5 is the annual SEC filing that catches exempt transactions insiders didn't have to report on Form 4 during the year. Here's what goes on it and when it's due.
• What is a Schedule 13D SEC filing? (Activist ownership disclosure)Schedule 13D is filed when an investor crosses 5% beneficial ownership with intent to influence or change control. Here's what's in it and why it moves stocks.
• What is a Schedule 13G SEC filing? (Passive 5%+ ownership disclosure)Schedule 13G is the simplified SEC filing for passive investors who cross 5% beneficial ownership. Here's who qualifies, when it's due, and when a 13G must convert to 13D.
• What is a 10-Q filing? (Quarterly report explained)A 10-Q is the unaudited quarterly financial report that public companies file with the SEC within 40-45 days of each quarter end. Here's what's in it and how to use it.
• What is a Form S-1 filing? (IPO registration statement explained)An S-1 is the SEC registration statement required before a company can sell securities to the public in an IPO. Here's what it discloses and how the process works.
• Form 4 vs Form 144: what's the difference?Form 4 is filed after an insider transaction. Form 144 is filed before a planned sale of restricted or control securities. Here's how they differ and when both apply.
• Schedule 13D vs 13G: what's the difference?13D is the activist filing; 13G is the passive holder filing. Both are triggered by crossing 5% ownership, but the deadlines, content, and implications are very different.