Direct answer
The spread in major currency pairs is the difference between the buy (ask) and sell (bid prices you see. It is affected by (1) how much liquidity exists in the market for that currency pair, (2) how volatile or active price action is, (3) how orders are executed and matched to prices, and (4) broker-specific pricing and cost policies that determine how total trading costs show up in the quote.
Mechanism and definition: what “spread” really measures
A spread is a market micro-cost: you pay the spread when you enter, because you buy at the ask and sell at the bid. For a major pair, the underlying currencies trade in deep, global markets, but “deep” does not mean “unchanging.” Liquidity can thin during certain hours, around major news, or when participants reduce risk. When fewer willing counterparties exist at the quoted prices, the dealer or pricing system may need a larger buffer—so the spread can widen.
A second layer is how the broker’s system turns market conditions into displayed quotes and fills. Two brokers can show different spreads at the same time because they may handle orders differently (for example, how they route orders, how they handle partial fills, and how they reflect internal costs). Even if the “raw” market is similar, the broker’s execution design affects the final cost you experience.
Finally, broker policies influence the cost mix. Some brokers quote a wider “headline” spread but charge lower commissions; others do the opposite. This matters because the total transaction cost depends on both the spread and any additional fees. Also, brokers may impose or reflect constraints such as minimum order sizes or how they treat market conditions during fast price changes.
Evidence or example (with clear assumptions)
Assume a major pair has strong liquidity most of the day, so many participants are ready to trade near current prices. In that setting, the bid and ask quotes can be relatively close because counterparties are likely to accept trades promptly.
Now change one variable: assume volatility spikes. Even in major pairs, rapid movement increases the risk that a quoted price becomes stale before the order can be matched. If a pricing system cannot update quotes quickly enough, it may widen the spread to reduce the chance of an unfavorable fill.
Assume a third change: execution becomes harder. For example, if a broker’s process prioritizes matching internally or applies routing that results in more uncertainty about the next available price, then the broker may widen the displayed spread or apply stricter handling during short-term dislocations. The key idea is not which approach is “better,” but that execution pathways can change the cost you observe.
Limitations and risks (material failure modes)
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Spreads are not stationary. Even for major pairs, spreads can widen suddenly during fast markets, when liquidity thins, or when quotes update more slowly. A “typical” spread does not guarantee the future spread.
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Displayed spread may not equal total cost. If commission exists, two quotes can look similar but lead to different total expenses. Conversely, a larger spread with low commissions can be cheaper than a smaller spread with high fees.
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Timing and order size matter. Larger orders or orders placed during thin liquidity can experience worse fills than what you saw on a smaller reference quote.
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Execution risk can appear as pricing differences. In fast moves, slippage and partial fills can make your realized cost differ from the momentary displayed bid/ask.
Verification and next question
To verify how these factors matter for a specific situation, compare: (a) how spreads behave during calmer versus volatile periods, (b) whether additional commissions or fees exist alongside the quoted spread, and (c) how the broker explains its order handling and pricing approach in its general documentation. A good next question is: “Does my total transaction cost depend mostly on spread, commissions, or execution quality under fast market conditions?”