Ask a futures trader when the session starts and you will get at least three defensible answers. The contract trades close to around the clock, the pauses are short, and the word open is being asked to do work it cannot do unaided. Before a breakout rule can be written down, it has to say which open it means, because the ranges produced by each candidate describe genuinely different things.
Three Candidates, Not One
The first candidate is the electronic reopen after the daily maintenance break, which is where the trading day formally begins. The second is the point at which European participation arrives in volume. The third is the cash equity open, which for index products is the moment the underlying market itself starts trading.
Each produces a measurable range in its first thirty minutes and each range is real. They are simply measuring different populations of participants doing different things, and a level derived from one carries no particular authority over the other.
What the Overnight Range Actually Measures
Overnight volume is thin and unevenly distributed across the hours. A range formed then is often the product of a small number of participants, some of whom are hedging rather than expressing a view. Price can travel a long way on very little activity, and it can also sit almost still for hours because nobody with size is present. Both outcomes produce a range you can draw lines around, and neither says much about what the day will do once larger participants arrive.
What the overnight range is good for is context. It marks where the instrument has already been while most of the domestic market was away. If the cash open arrives and price immediately leaves that territory, something changed at the open. If the first hour of the cash session stays inside the overnight range, the open added nothing new and the earlier extremes are still the levels doing the work.
The Cash Open Concentrates Everything
The cash equity open differs in kind, not merely in degree. Orders accumulated overnight execute, index products reprice against an underlying that is finally trading, and participation arrives in a burst rather than a trickle. A range formed in that window is formed by a genuine crowd, which is what makes its edges meaningful: they were reached and rejected by real size rather than drifted through in the quiet.
This is the practical reason most opening range work on index futures uses the cash open rather than the electronic one. It is not convention for its own sake. It is that the edges are defended levels rather than incidental extremes, and a breakout rule depends entirely on the edge meaning something to somebody.
Levels That Survive the Handover
The interesting sessions are the ones where the two ranges disagree. Price spends the overnight hours building a range, then the cash open pushes straight through one of its edges and builds a new, smaller range on the far side. You now hold two sets of lines pointing in different directions.
The overnight extreme that was broken should not be discarded. It has become a level with history behind it, and price returning to it later in the session is a different event from price wandering through an unremarkable price. Treating the cash range as the tradeable structure while keeping the overnight extremes on the chart as reference is usually more useful than picking one and deleting the other.
Picking One and Keeping It
Whichever you settle on, the cost of switching exceeds the benefit. A trader who uses the cash open on quiet days and the overnight range on days when the overnight looks more interesting does not have a rule, only a series of judgements made with part of the outcome already visible. The record that accumulates cannot be evaluated, because it is a record of several different strategies interleaved.
Write down which open you measure, and write down the exception if you want one, for instance that a holiday shortened cash session is skipped rather than measured differently. An exception stated in advance is part of the rule. An exception discovered at the moment it happens to be convenient is not.