The concept of support and resistance in the charts is basic to the understanding of price patterns and their implications.
Edwards & Magee defined support as the "buying, actual or potential, sufficient in volume to halt a downtrend in prices for an Appreciable period." Resistance, of course, is the antithesis of this and consists of selling, actual or potential, in sufficient volume to keep prices from rising for a time. "Support and resistance, as thus defined, are nearly but not quite synonymous with demand and supply, respectively."
Further expounding this concept, Edwards & Magee tell us:
"A support level is a price level at which sufficient demand for a stock appears to hold a downtrend temporarily at least, and possibly reverse it. i.e., start prices moving up again. A resistance zone by the same token, is a price level at which sufficient supply of stock is forthcoming to stop, and possibly turn back, its uptrend. There is, theoretically, a certain amount of supply and a certain amount of demand at any given price level... But a support range represents a concentration of demand, and a resistance range represents a concentration of supply."
Support and resistance - in their basic forms - are represented on the charts as follows:
In a trending market, especially one in which prices travel within the confines of a clearly defined channel, the support and resistance lines will tend to keep prices within the channel, bouncing from support to resistance in an alternating "zigzag" pattern.
Support and resistance are more than just an upward trending or downward trending channel lines. They may be encountered from a variety of chart patterns and other places of price congestion on the charts.
One rule of thumb for determining where a market or security will meet with either support or resistance on the charts is to find previous chart areas where consolidation has occurred. If, for example, a particular stock has stalled out in a net sideways or other congestion pattern at a certain level in the recent past before falling to a lower level, it is all but likely that the stock will encounter difficulty in penetrating that same level later on as it rallies and tries to overcome it. This, of course, does not necessarily mean the former area of consolidation (in this case, resistance) will prove impenetrable; to the contrary, it will probably be overcome eventually. But not without considerable effort on behalf of the buyers. The greater the congestion, the greater the effort required to overcome that congestion, whether it is in the form of support or resistance. Thus support and resistance serve as checks in the development of a trend (be it a rising or a falling trend) to keep the trend from moving too far, too fast and thus getting out of hand and eliciting violent reactions. (This does not apply, of course, in market crashes or "buying panics," in which case support and resistance levels become meaningless. But such instances are fortunately quite rare.
This leads us to the next related principle of support and resistance which Edwards & Magee elucidate for us: "...here is the interesting and the important fact which, curiously enough, many casual chart observers appear never to grasp: These critical price levels constantly switch their roles from support to resistance and from resistance to support. A former top, once it has been surpassed, becomes a bottom zone in a subsequent downtrend; and an old bottom, once it has been penetrated, becomes a top zone in a later advancing phase."
Thus, if a certain security breaks through an overhead resistance level at, say $50, then the moment prices are above the$50 level, it automatically becomes a support. Conversely, if the $50 in our hypothetical security had been a support checking prices from moving below it and the $50 level is suddenly penetrated then $50 automatically becomes resistance. This principle, which we call the "principle of interchangeability", hold true for older levels of support and resistance as well, not just recent levels.
Other instances of support and resistance can be found not only in areas of chart congestion but in geometric chart patterns as well. The symmetrical triangle affords just such an example. Throughout the formation of the triangle, the upper and lower boundary lines serve as resistance and support, respectively. However, an even stronger level of support level of support and resistance (depending on which direction prices take upon breaking out from the triangle) is provided by the apex of the triangle. By drawing a horizontal line from the apex and extending it across the chart an analyst will be provided with a reliable support/resistance level. However, such levels usually become weak as time passes. Thus, a chartist will want to regard this as a strong support/resistance only in the days/weeks immediately following a price breakout from the triangle.
Concerning volume, it is sufficient merely to point out that the power of a resistance (or support) range is estimated by using the criterion of volume. In other words, the greater the amount of volume was recorded at the making of a top (resistance) or bottom (support) in a given market or security, the greater the strength of that top or bottom will be and the more effort will be necessary to penetrate it in the future. As Edwards & Magee put it:
"In brief, a single, sharp, high-volume bottom offers somewhat more resistance than a series of bottoms with the same volume spread out in time and with intervening rallies."
Another criterion Edward & Magee discuss that is worth noting here is the extent of the subsequent decline from a resistance zone. Or, to phrase it differently, how far will prices have to climb before they encounter the old bottom zone whose resistance potential the analyst attempt to appraise? "Generally speaking," Edwards & Magee write, "the greater the distance, the greater the resistance."
In other words, the higher that prices must travel before breaking the previous top, the stronger the resistance that top is likely to hold.
Finally, in answer to the oft-asked question as to what exactly constitutes a legitimate "break" of either support or resistance, we would refer the analyst back to the old Edwards & Magee "three percent rule," which states that a break above a support or resistance level (or through a corresponding chart pattern) by distance of at least 3 percent, and accompanied by increased trading volume, should be viewed as the start of a new trend and therefore followed.