Waterfall — Plan First, Build Once

The V-Model is a Waterfall variant that pairs each left-hand phase with a corresponding test phase on the right (unit ↔ code, system test ↔ design, acceptance ↔ requirements).

• Requirements are genuinely fixed — regulatory submissions, hardware integrations, contract deliverables.
• Cost of late change is enormous — flight software, medical devices, civil engineering.
• Multi-vendor handoffs where a written spec is the legal interface.
• Fixed-bid procurement — government RFPs that require a Statement of Work.

• Late feedback. Users see software near the end — by then half the requirements are wrong.
• Big-bang integration risk. All the modules meet for the first time during test phase, and break.
• Change is expensive. A late requirements shift cascades back through every signed document.
• Documentation theater. Hundreds of pages of specs nobody re-reads.
• Hidden iteration. Real Waterfall projects almost always loop back covertly — managed badly because the process pretends they don't.