What is a technical indicator, really?
Open any charting platform and you'll find a menu of a hundred indicators with confident names: Relative Strength Index, Chaikin Money Flow, Awesome Oscillator. The menu implies a machine room full of instruments, each measuring something new. That is not what is happening.
A technical indicator is a formula applied to price and volume data that produces a new series of numbers. Input: open, high, low, close, volume — the same OHLCV you learned to read as candles in Charting Basics. Output: another line, drawn on the price panel or in a pane below it. Nothing else goes in.
The one sentence that keeps you honest
An indicator cannot contain information that isn't already in the price and volume history. It is a transformation, not a sensor. A thermometer measures something genuinely new about a room; a moving average only rearranges numbers you already had.
That sounds deflating. It isn't. Rearrangement is useful: it makes questions answerable at a glance that raw candles answer only with effort. "Is today's move fast by this stock's own recent standards?" is tedious to eyeball and trivial once computed. An indicator is a calculator you are allowed to look at — not an oracle you are supposed to obey.
The four families
Nearly every indicator on that hundred-item menu belongs to one of four families, and each family answers one descriptive question:
- Trend (moving averages, MACD) — which way has price been leaning, once daily noise is averaged out?
- Momentum / oscillators (RSI, stochastics) — how fast, and how one-sided, have recent moves been?
- Volatility (Bollinger Bands, ATR) — how much is this instrument moving around — in points, or in standard deviations?
- Volume (OBV, VWAP) — how many shares backed those moves, and at what prices did they actually change hands?
Knowing the family tells you most of what an unfamiliar indicator does before you read a single formula.
Overlays and oscillators
Two ways they are drawn. Overlays share the price panel because they are in price units — moving averages, Bollinger Bands. Oscillators get their own pane and their own scale, often bounded between 0 and 100 — RSI, stochastics. It's only a display distinction, but it's the first thing you notice, and it quietly tells you whether the output is comparable to a price.
A tiny worked example
Take five closes: 100, 102, 101, 105, 107. Their sum is 515, so the five-day average is 103. Tomorrow's close is 109: the 100 drops out of the window, the 109 enters, the sum becomes 515 − 100 + 109 = 524, and the average becomes 104.8.
Notice what just happened. Information was deliberately thrown away — which day was which, how violent each session was — in exchange for one legible number. Every indicator makes that trade: detail for legibility. What it throws away is exactly where its blind spots live.
Those five closes are the middle of the series below, marked. It is the whole input to the calculation: no volume, no dates, no news.
The framing this course keeps
Each lesson describes what an indicator measures and what traders watch. "RSI crossed 70" is an observation about the last fourteen sessions. The leap from there to "therefore sell" is a claim about the future that the arithmetic does not support, and this course will never make it for you. Tickers here are illustrations, not suggestions.
In the data
Every line on a chart is a recipe with three parts: the transformation (a 20-day average, a 14-day RSI), its window, and the price series it is fed. The chart below carries two, and the legend under it names all three parts, including the series: adjusted closes.
The third part is the one people forget. Every new dividend rescales the whole adjusted history backwards, so the same recipe over the same dates gives slightly different numbers after the next ex-dividend date. The transformation is fixed; the series underneath it is not.
Try it now
- Average the five marked closes on the schematic above by hand. That number, and the operation that produced it, is an indicator in full — there is nothing else in the box.
- Now the same transformations on real prices. The chart below carries a 5-day average over the price and a 14-day RSI in a pane of its own. Read the latest value of each and write down its units: one is in dollars and belongs on the price axis, the other is on a 0–100 scale and could not be drawn on it at all, which is why it sits underneath on its own axis. That is the overlay-versus-oscillator distinction, and it shows up in the numbers before it shows up on any chart.
- Name the family of each and write down the descriptive question it answers. If you can't phrase the question, you don't yet know what the line measures — which is the honest starting point for this whole course.