Commissions are visible, and these days often zero. The spread is invisible and never zero, and across enough trades, it's the bigger cost of the two.
At any given instant there are two prices for a security, not one: the price someone will pay to buy it from you, and the higher price someone will charge to sell it to you. The gap between them is the spread, and it's the most consistently ignored cost in investing, mainly because no one ever sends a bill for it.
The mechanism is simple, and there's nothing sinister about it. The people standing ready to trade, who make it possible to buy or sell whenever you feel like it, need to be paid for the risk of doing so. They get paid by buying slightly low and selling slightly high, and the gap is their income. The spread, in other words, is the price of being able to transact instantly, and that convenience has real value.
For an investor, this means every trade starts in a small hole. Buy a security and sell it back immediately, with no change in the market at all, and you get back less than you paid. That difference is the spread. It happens whether or not any commission was charged, and it happens at every brokerage, because the spread isn't a fee your broker invented. It's baked into the market itself.
That matters a great deal given a change most investors treat as an unambiguous win: commissions at many brokerages have dropped to zero. The visible cost of trading disappeared, so the obvious conclusion is that trading is now free. It isn't. The spread is still there, it's paid on every trade, and it's disclosed in a way almost nobody actually notices. Making a cost invisible doesn't make it smaller.
It's worth understanding, rather than just assuming, how zero-commission brokerages make their money. Under some arrangements, brokers get paid to route customer orders to particular firms for execution. Whether that arrangement produces worse prices for customers is a genuinely contested question, and this piece isn't going to settle it. What can be said plainly is that the service isn't free, that the brokerage is getting paid by somebody, and that an investor who thinks they're getting something for nothing simply hasn't found out where the payment is coming from.
The size of the spread varies enormously, and the variation itself tells you something. In the shares of huge companies, traded constantly by huge numbers of people, the spread is a tiny sliver of the price, negligible for anyone holding long term. In smaller companies, traded rarely, the spread can be substantial, and an investor buying and selling one can lose a meaningful share of their capital just on the round trip. The spread is, in effect, a measure of how much competition exists to serve you.
That produces a direct relationship between the spread and how often you trade, which is really the whole point. Buy once and hold for twenty years, and you pay the spread exactly twice; over that stretch, its effect on your outcome is negligible. Trade weekly, and you pay it a hundred times a year, and the accumulated cost becomes a real drag on returns, regardless of whether any of those hundred decisions were good ones. The spread is a tax on activity, charged whether or not the activity produced any insight at all.
This is a big part of the mechanism behind the research on trading frequency examined elsewhere in this project. When studies find that the most active traders earn the least, and that costs explain much of the gap, the spread is a substantial piece of what's being described. It isn't that active traders picked badly. It's that they paid the spread again and again, and the accumulated payments ate up whatever edge they might otherwise have had.
There are practical, modest steps available. Trade more liquid securities and the spread narrows. Trade during the busiest stretch of the session, when more participants are around, rather than at the open or close when the book is thin, and the spread narrows further. Set a limit on the price you'll accept, and you're protected from getting filled at a bad point in a wide market. None of this is dramatic. Together, it chips away at a cost that would otherwise get paid in silence.
The most powerful lever, though, is simply trading less. Every technique for cutting the cost of a trade is secondary to the option of not making the trade. An investor who trades less often hasn't just cut their spread costs. They've cut their commissions, their taxes, and their chances to make a mistake, and they've done it without needing any skill whatsoever.
There's a broader principle buried in here. When something that used to cost money explicitly becomes free, the payment hasn't vanished, it's just moved somewhere less visible. That's true of a lot of arrangements an investor runs into, and the useful habit isn't suspicion, just curiosity: if this is free to me, who's paying, and to whom, and for what? The answer is usually there for anyone who looks, and it's usually mundane rather than scandalous. What matters is knowing the answer instead of assuming none is needed, because a cost you haven't identified is a cost you can't manage.
VESTFY™ presents the spread as the clearest example available of a cost that got hidden rather than eliminated. An investor who believes their trading is free hasn't looked closely at where the money actually goes, and believing an activity is costless is precisely the belief that gets people doing far more of it than serves them.