What is the spread?

The spread is the distance between the highest bid and the lowest ask. Cross it and you have paid a cost that never appears on your statement as a fee.

Why it matters

Some venues advertise zero-commission trading and earn from the spread instead. That is not necessarily worse, but it makes the true cost invisible and hard to compare — a wide spread with no fee can easily cost more than a tight spread with one. When comparing platforms, look at the total cost of a round trip on the pair you actually trade, not at the advertised commission. Spreads also widen sharply during volatility and on thinly traded assets, exactly when you are most likely to be trading.