US30SIGNALS Price-weighted does not mean “add the prices and stop”
The Dow Jones Industrial Average uses price weighting. Its level is based on component prices divided by an index divisor. S&P Dow Jones Indices explains that divisor adjustments preserve continuity when corporate actions would otherwise create a non-market jump in an index.
A two-for-one split halves the quoted price per share and doubles the share count, before any separate market move. In a price-weighted calculation, the price change matters mechanically even though the split itself has not halved the shareholder's holding value.
A three-stock example makes the adjustment visible
This is a fictional teaching index, not the actual Dow or its divisor. Suppose three stocks cost $400, $100 and $100. Their price sum is $600. With a divisor of 1, the index level is 600.
| Stage | Price sum | Divisor | Index level |
|---|---|---|---|
| Before the split | $600 | 1 | 600 |
| After a 2-for-1 split, no adjustment | $400 | 1 | 400 — artificial drop |
| After the split, adjusted divisor | $400 | 2/3 | 600 — continuous |
The new divisor is 400 ÷ 600 = 2/3. Dividing the new price sum by that divisor restores the same index level. The arithmetic removes the split's mechanical effect; it does not cancel genuine buying or selling in the stock.
Why the stock can have less influence afterwards
Before the split, the $400 stock represented 400 ÷ 600, or about 66.7%, of our teaching index's price sum. Afterwards, the $200 stock represents 200 ÷ 400, or 50%. The index level is continuous, but the weights are different.
A subsequent 10% rise in that stock would add $20 to the new price sum, moving the index by 20 ÷ (2/3) = 30 points. Before the split, a 10% rise would have added $40 and moved the index by 40 points. This is a comparison of percentage moves, not a statement that every one-dollar move has less influence.
S&P's index-literacy explanation distinguishes price weighting from market-cap weighting and explains why split adjustments differ between them.
What this means for a US30 chart
A chart showing a constituent's price halving after a split is not, on its own, evidence of a matching index sell-off. Separate the corporate action from the market's response and the broker's own US30 contract treatment.
This article explains the underlying index mechanism. It does not specify a broker's financing adjustments, executable quote or lot value. Those belong to the particular product. The US30 guide and point-value page below connect the index explanation to instrument terminology and position arithmetic.