Compare single life, joint first-to-die and joint last-to-die coverage: who's insured, when the benefit pays, and how the structure moves the premium.
The policyholder owns the contract; the life insured is the person the payout depends on. A single-life policy insures one person and pays if that person dies during the term. A joint first-to-die policy insures two lives for one benefit and pays on the first death; that suits two people sharing a debt, like Sue & Cindy's $250,000 mortgage, so the survivor can pay it off. A joint last-to-die policy pays only on the second death: a natural fit for a tax bill that isn't due until the second spouse dies, like Hamish & Elizabeth's cottage.
The curves show the illustrative chance each structure has paid by a given age (built from a simplified mortality curve). First-to-die rises fastest (two lives, either death triggers it), so it costs the most per dollar of coverage, but still less than two separate policies because the insurer only ever pays one benefit. Last-to-die rises slowest (both people must die first), so it's the cheapest, and because coverage is usually needed for life it's typically sold as permanent insurance rather than term.