If you responded a couple of months ago to one of those mailshots urging you to invest £7,000, your yearly entitlement for an equity ISA, in a tracker fund you probably supposed that your money would be spread evenly across the market.
It would not be greatly exposed to the fortunes or misfortunes of any individual company.
You may be surprised, then, to learn that if you chose the Footsie All-Share, about £1,900 of your investment went into just four shares - BP, HSBC, Vodafone and GlaxoSmithKline. This is because the index and the funds tracking it are weighted according to the market value of the companies in it.
The precise amounts vary from day to day, but on Tuesday night these four accounted for 26.85 per cent of the All-Share, and more of the 100-share Footsie. BP alone was worth 8.2 per cent of the All-Share.
This doesn't mean you made a bad investment. These companies have got big by being successful. But it may be a more concentrated investment than you thought you were buying.
I don't suppose for a moment that President Putin will ever confiscate BP's colossal investment in Russian oil.
But if he does it will cost you money you will notice. If you accepted that risk - and wanted the great potential benefit that goes with it - you would have bought BP shares directly, not a tracker.
The anomaly will become more acute in July when Shell merges with its sister company Royal Dutch. The combined entity will be worth about seven per cent of the All-Share, instead of Shell's present three per cent. BP and Shell together will have 15 per cent.
Tracker funds will have to dump other shares to plunge huge sums into the enlarged Shell - in a market that can see them coming.
Many pension funds, which want to spread their investments more evenly, took avoiding action some time ago, often by switching into overseas equities so that they have more big companies to choose from.
This did British share prices no good at a time when pensions were running down their equity holdings anyway for the sake of greater certainty in fixed interest investments.
Nor was it an unqualified success. Many of the biggest overseas companies are American, so funds that switched into them suffered when the dollar fell.
The good news is that the coming Shell merger has concentrated minds at FTSE, the company that runs the indices. They have devised new 100-share and All-Share measures, just the same as the present ones, except that no company can be worth more than five per cent of the whole.
These should start running on June 20 - before nimble investors can make an easy killing out of the Royal Dutch/Shell affair at the expense of tracker funds.
The present indices will go on as they are. There will just be more choice. New "capped" trackers will doubtless spring up for those who don't want more than five per cent of their money in any single company.
Which will do best? I don't know, nor does anybody else. But I do know which will limit the risk.