What it means
The order sits in a family of conditional instructions that attach a price rule to a trade. Instead of saying buy at whatever the market offers, the instruction says buy only if the next available price is lower than the last one printed.
Brokers and exchanges apply the rule mechanically. In practice the execution must happen at a price below the last sale, or at the same price as the last sale if that last trade was itself a move downwards.
That second part stops the order from being blocked forever in a market that is drifting down in small steps. The commercial reason to use it is market impact.
A large buy order placed at the market pushes the price up against the buyer, so splitting the purchase into instructions that only fill on weakness keeps the average entry price lower. Fund managers building a position over several days use this kind of order to avoid bidding against themselves.
The trade-off is the risk of never being filled. If the stock rises steadily all day the order simply sits there, and the buyer watches the price they wanted disappear.
For anyone who must own the shares by a deadline, such as a fund tracking a change in an index, this is the wrong tool. A buy minus is not the same as a limit order, although the two are often confused.
A limit order names a fixed price ceiling, while a buy minus names a relationship to the last traded price, so its effective ceiling moves with the market. On a fast-moving day that difference can be several percentage points.
Electronic markets have made the instruction rarer than it once was, because trading algorithms now handle this kind of logic in far more flexible ways. The concept still matters as the simplest example of an order that gives up certainty of execution in return for a better price.
In practice
Real-world examples.
Example
A pension fund is adding 300,000 shares of a mid-sized engineering group to its portfolio over a fortnight. The dealer uses buy minus instructions each morning so the fund only buys on dips, and it ends the fortnight with an average price three cents below the period's average traded price. On that size the discipline is worth about $9,000.
Example
A company running a share buyback programme instructs its broker to buy only below the last print, so the company is never seen chasing its own share price up. The finance director can then tell the audit committee that the programme added no upward pressure of its own.
Example
A family office wants a holding in a thinly traded water utility but is not in a hurry. It leaves buy minus instructions with its broker for six weeks and accumulates the position slowly, accepting that on quiet days nothing at all gets done.
Formula
Calculation
Price saving from a buy minus order = (price a market order would have paid - average price actually paid) x number of shares
A fund wants 40,000 shares of a company trading at $25.00. A single market order would have filled the whole block at around $25.10 because of its size, costing 40,000 x 25.10 = $1,004,000. Instead the manager places buy minus instructions over two days and fills all 40,000 shares at an average of $24.80, costing 40,000 x 24.80 = $992,000. The saving is 1,004,000 - 992,000 = $12,000, which is 12,000 / 1,004,000 = 1.2% of what the order would otherwise have cost.Case study
Seen in the real world.
Harrowgate Capital is an illustrative, fictional equity fund with $300,000,000 under management. Its dealing desk reviewed a year of trades and found that market orders placed in the first hour of trading had cost the fund an average of 0.4% more than the day's average price, simply because the desk was buying into early momentum.
The head of dealing changed the standing instruction for all non-urgent purchases to buy minus, with a rule that any position not completed in five days reverted to a normal working order. Fills became slower and roughly one order in six went unfilled, which the portfolio managers disliked.
Over the following year the desk measured the average entry price against the daily average and found a saving of about 0.25%, worth roughly $180,000 on the fund's turnover. The illustrative point is that execution style is a real source of return, and that it is only visible once someone measures it.
Watch out
Common mistakes.
- Treating a buy minus order as a guarantee of a lower price, when its real effect is to make execution conditional and therefore uncertain.
- Using it for a trade that has a hard deadline, such as matching an index change, where failing to be filled is a much bigger problem than paying a few cents more.
- Confusing it with a limit order, and so assuming there is a fixed price ceiling when the reference point actually moves with every print.
Questions
People also ask.
Does a buy minus order ever execute at the same price as the last trade?
Yes, it can when the last trade was itself a downward move, which stops the order from being stuck in a steadily falling market.
Is this order type useful to a small private investor?
Rarely, because on small sizes the market impact being avoided is almost nothing, and the risk of missing the trade outweighs a few cents of saving.
How would a finance team know whether the order type helped?
Compare the average price paid against the volume weighted average price for the same period, and look at how many orders went unfilled, because both numbers are needed to judge it fairly.
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