Support and resistance (S/R) are price levels — more honestly, price zones — where buying or selling pressure has repeatedly been strong enough to halt or reverse a move, and so are expected to react again. Support is a level where demand has been firm enough to stop a decline; resistance is a level where supply has been firm enough to cap an advance (StockCharts ChartSchool). The concept is the foundational framework of charting — almost every other tool (trendlines, breakouts, patterns, Fibonacci) is ultimately a way of locating where supply and demand have shifted. Its core tension is that it is simultaneously the most universally watched idea in technical analysis and one of the most subjective: a "level" is really a fuzzy band, where you draw it is a judgment call, and part of why levels work at all is that so many people watch the same ones — making S/R partly self-fulfilling rather than an independent property of the asset. This node defines the family and routes to its members; the operational specifics live in the children.

The family (route to children)

• Horizontal Levels — static price levels drawn from prior swing highs/lows, congestion zones, and reaction points. The canonical, most widely used form. → Horizontal Levels
• Dynamic S/R (Moving Averages) — sloping levels that track price over time; moving averages (e.g. 50- and 200-day) and trendlines act as support/resistance that moves with the trend. → Dynamic S/R (Moving Averages)
• Round Numbers & Psychological Levels — whole-number prices (10.00, 100, 5,000 on an index) that attract order clustering. This is the one sub-area with genuine academic support (see below). → Round Numbers & Psychological Levels
• Role Reversal (Support becomes Resistance) — the polarity principle: once price decisively breaks a level, the broken support tends to act as resistance on a retest, and vice versa. → Role Reversal
• Supply & Demand Zones — a related but distinct lineage (Sam Seiden / order-flow framing): rectangular zones marking the origin of a sharp move, read as areas of stacked unfilled orders rather than a single line. → Supply & Demand Zones

How it's used in practice

Traders use S/R for four jobs: (1) entries — buying near support / shorting near resistance in a range; (2) stops — placing protective stops just beyond a level, on the logic that a clean break invalidates the thesis; (3) targets — taking profit into the next level overhead/below; and (4) breakout trading — entering when price decisively clears a level, often expecting role reversal to confirm the new direction. Because a level is a zone, practitioners distinguish a wick/probe through a level from a closing break (ideally on a higher timeframe), and many require a volume expansion or a retest-and-hold before trusting a break. The more times a level has been tested and the more recent and higher-volume the tests, the more weight it is given.

Adoption, debate & evidence

S/R is near-universal across retail and professional discretionary trading, and is embedded in the CMT curriculum and every charting platform. But the rigorous predictive evidence is thinner than its ubiquity suggests, and it splits sharply by sub-type:

• General S/R levels: The strongest direct evidence is Carol Osler's Federal Reserve Bank of New York work on intraday FX. Support for Resistance (2000) tested published S/R levels from six FX firms and found they had statistically significant power to predict intraday trend interruptions, though the strength varied by firm and currency pair and the edge persisted only several business days. This is real evidence — but it is intraday FX order-flow, not a broad cross-market endorsement, and should not be over-generalized.
• Round numbers / psychological levels: This is the best-supported sub-area. Osler's Currency Orders and Exchange-Rate Dynamics (FRBNY, 2003) examined a real FX order book and documented heavy clustering of orders at round numbers — take-profit orders cluster at round numbers (creating a reflecting/bounce effect) while stop-loss orders cluster just beyond them (creating cascades on a break). In equities, Donaldson & Kim (1993) found support/resistance behavior in the Dow around round-hundred levels, building on Osborne (1962) and Niederhoffer (1965) on price clustering. The mechanism is concrete and order-flow-based — though evidence is mixed elsewhere (Dorfleitner & Klein 2009 could not confirm barriers in several European indices), so it is not universal.
• The self-fulfilling caveat: Much of S/R's apparent power is reflexive — clustered orders at watched levels create the very buying/selling that makes the level hold (Osler's order-book finding). That is a genuine market microstructure effect, but it also means S/R is closer to a map of where reactions are likely than a law of where price must turn.

Strengths & limitations

S/R's strength is that it is a context map: it tells you where a move is more likely to pause, fail, or accelerate, which is exactly what you need for stop and target placement. Its limitations are equally real. It is subjective — two analysts draw different levels on the same chart, and a level is a zone, so "it held" / "it broke" is often decided after the fact. It is not a precise trigger — false breaks (stop-runs / liquidity sweeps) and overshoots are routine, which is why level-based stops get hunted. It is regime-dependent — S/R reasoning works best in ranges and orderly trends and degrades in gap-driven, news-driven, or thin markets. The single most common misuse is treating a level as an exact line and a guaranteed bounce — entering reflexively at a round number or prior high without confirmation, no volume context, and a stop placed at the obvious spot everyone else uses.

System relevance

S/R is upstream context for the Augustus trade-setup agent, which consumes levels to reason about entry proximity, stop placement (beyond the nearest invalidating level), and reward-to-risk versus the next level. The hard caveat to pass downstream: a "level" is a zone, not a number, and S/R is partly self-fulfilling — Augustus should treat S/R as a probabilistic reaction map combined with confirmation (volume, closing breaks, retest behavior) and let Cairn's measured track record, not the chart's apparent obviousness, calibrate how much weight any single level earns. Swing-specific entry/stop/target mechanics belong to the Swing Trading branch — cross-link there rather than duplicating.

Sources

• StockCharts ChartSchool — Support and Resistance (definitions, polarity principle, zones vs. lines): https://chartschool.stockcharts.com/table-of-contents/chart-analysis/support-and-resistance
• Carol L. Osler (2000), Support for Resistance: Technical Analysis and Intraday Exchange Rates, FRBNY Economic Policy Review.
• Carol L. Osler (2003), Currency Orders and Exchange-Rate Dynamics: An Explanation for the Predictive Success of Technical Analysis, FRBNY Staff Report 125 (order clustering at round numbers; take-profit vs stop-loss placement): https://www.newyorkfed.org/medialibrary/media/research/staff_reports/sr125.pdf
• Donaldson & Kim (1993), Price Barriers in the Dow Jones Industrial Average (round-hundred S/R behavior); Osborne (1962); Niederhoffer (1965) — price clustering origins.
• De Grauwe & Decupere (1992); Dorfleitner & Klein (2009) — psychological-barrier evidence (mixed across markets).
• Investopedia / general practitioner literature; Sam Seiden — supply & demand zone framing (vs. horizontal S/R).