An envelope is
generally two moving averages, one adjusted to be above price, the
other adjusted to be below. Typically, an envelope defines the limits
of a security's normal trading range. In use, traders tend to trade
reversals using the bands - e.g. if price penetrates the upper band
they go short, if it penetrates the lower band they go long. The
definition of the band depends to a large extent on volatility,
which is why Bollinger bands are so useful - they self-modify as
volatility expands and contracts. Envelopes can employ simple moving
averages, exponential moving averages, or other more complicated
averaging strategies. Day traders make extensive use of envelopes,
and in fact you could argue that the SureFireThing Camarilla Equation uses 'reversion
to mean' theory to generate a number of pseudo-envelopes which sit
around a nominal center point providing incredibly useful day trading
signals (although they also provide breakout points, not just reversals).
They tend to work best on liquid stocks, as they track the market's
expectations, and the more liquid a security, the closer it approaches
ideal behaviour..
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