The cross whose intersection sets the price. Two blades of one scissor — demand sloping down, supply sloping up — meet at the single point where quantity demanded equals quantity supplied. Push a curve and the point moves; drop a tax between the blades and it splits by elasticity. Rendered, not quoted.
source Marshall, Principles of Economics, Book V (1890) — equilibrium of normal demand and supply. No single canonical DOI; full text at marxists.org/reference/…/marshall amber — a MODEL of a market, not investment advice.
Linear market. Demand Qd(p)=A−B·p falls with price; supply Qs(p)=C+D·p rises with it.
A=100 B=2 C=10 D=3
Equilibrium is the price p* solving Qd(p*)=Qs(p*) — the numeric root of excess(p)=Qd−Qs. Because demand strictly falls and supply strictly rises, excess is strictly decreasing, so the root is unique: the curves cross exactly once.
Analytic: p*=(A−C)/(B+D)=18, Q*=64.
Marshall 1890 fused two half-drawn curves into one cross and standardised it. The equilibrium Qd=Qs, its comparative statics, and tax incidence-by-elasticity feed directly downstream: the responsiveness measured here is exactly what the-price-elasticity quantifies — steeper (less elastic) side, larger share of any wedge.
Live re-check of the engine's invariants. Confirms green; flips red the instant the tamper (window 6) is applied and the single-crossing / upward-supply law breaks.
witness idledemand: A=100, B=2 (down)
supply: C=10, D=3 (up)
shift test: +10 tax: t=5
Live cross. Solid = current market; dashed violet = demand shifted right (+10). The dot is p*.
booting…—
A per-unit tax opens a wedge exactly equal to t; the buyer price rises, the seller price falls, and the split lands harder on the less-elastic side. Deadweight loss is the shaded triangle — trades that no longer happen.
The cross assumes what no market fully is. Perfect competition (price-takers, homogeneous good), rational agents, and ceteris paribus are idealisations amber. Real curves shift together, have kinks, memory (hysteresis), and expectations. Economics is not physics: "the law of demand" is a strong regularity, not a conservation law. Any forecast off this model is amber, not advice.
"Price is set by cost of production alone." — supply is only one blade; demand sets it jointly (Marshall's scissors).
"A tax is paid by whoever the law names." — statutory incidence ≠ economic incidence; the split follows elasticities, not the statute.
"A price ceiling just makes things cheaper for everyone." — below p* it creates a shortage: Qd>Qs, rationing, queues.
The disclosed planted void: flip supply to slope downward (like demand). Single-crossing / upward-supply is no longer guaranteed and the equilibrium logic breaks. Window 7 catches it live.