Direct answer
The spread in Sterling crosses is mainly affected by (1) liquidity, (2) volatility, (3) execution venue mechanics, and (4) provider/broker policy on how orders are routed and priced. In practical terms, the spread tends to be narrower when many counterparties are trading both legs efficiently, and wider when fewer quotes are available or prices move faster than market makers can hedge.
Mechanics: define “spread” for Sterling crosses
A spread is the difference between the best quoted buy price (ask) and the best quoted sell price (bid) for a given currency pair at a given moment.
A Sterling cross is a currency pair that includes the British pound (GBP) but does not pair GBP directly with the same “base” currency used in some major pairs. What makes crosses special for spreads is that the pair’s tradability depends on both currencies involved. Even if GBP itself is stable, a sudden change in the other currency (for example, higher uncertainty or fewer active participants) can reduce available liquidity and widen the quoted spread.
A useful way to think about a quoted spread is: when dealers or liquidity providers expect frequent trading and smooth price discovery, they can quote tighter. When they expect uncertainty, they often quote wider to manage the cost and risk of holding positions until the order is completed.
Evidence or example: how liquidity and volatility change the spread
Consider two simplified conditions for a hypothetical Sterling cross:
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High liquidity, low volatility: Many participants are posting bids and offers, spreads are updated frequently, and price moves are relatively small between quote updates. Under these conditions, a quoted bid/ask difference can remain tight.
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Low liquidity, higher volatility: Fewer market participants are actively quoting, and prices change quickly relative to how fast quotes are updated. As a result, the bid/ask gap usually widens because liquidity providers price in the risk that the market will move before they can offset (hedge) the exposure created by your trade.
A key point is that Sterling-cross spreads can react to events in either currency leg. If the non-GBP currency becomes harder to trade (wider market risk, fewer active quotes, or fragmented trading), the cross can show a wider spread even if GBP-specific conditions look unchanged.
Limitations and risks: what can fail in the spread story
Several material limitations are common:
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Quoted spread vs. execution outcome: Even when a quote shows a tight spread, large orders or fast-moving markets can cause slippage—you may fill at levels that effectively widen your cost. This is especially noticeable when available depth near the bid/ask is limited.
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Venue and handling differences: Execution can be affected by whether an order is filled against a liquidity pool, routed to external venues, or managed internally by a provider. Two providers can display different “effective” spreads for the same pair at the same time due to routing and fill logic.
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Rapid regime changes: Liquidity can shift quickly around major economic releases or sudden market repricing. A spread that looks stable can widen unexpectedly when quotes disappear or update slower than price.
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Time-of-day effects: Liquidity typically varies by trading session overlap, so spreads are often better during periods with more active trading and thinner during less-active hours.
These limitations mean you should treat spread as a dynamic market observation, not a stable property of the Sterling cross.
Verification and next question
To independently verify what is affecting spreads for Sterling crosses, compare how spreads behave under different, well-defined conditions:
- Track spreads during periods of relatively steady price discovery versus periods of rapid movement.
- Observe whether wider spreads coincide with reduced market activity or thinner quoted depth.
- Compare your observed execution costs (not just displayed quotes) for different order sizes.
- Note whether spreads change more during certain time windows, suggesting liquidity-driven effects.
If you want to narrow the question further, the next useful step is to ask what is driving liquidity at that moment: is it reduced participation in one currency leg, temporary quote withdrawal, or operational differences in how orders are matched?