Direct answer
In forex, the “spread line” is commonly referred to as the bid-ask spread. It is based on the bid price (what buyers are willing to pay) and the ask price (what sellers are willing to receive). The bid-ask spread is the difference between those two quote prices.
Explanation and mechanics
Forex quotes are typically shown as two numbers for the same currency pair: a bid and an ask.
- Bid: the price at which you could sell the base currency (in the market view of that quote).
- Ask: the price at which you could buy the base currency.
- Bid-ask spread: ask − bid.
When people say “the spread line,” they often mean the line or indicator on a trading screen that represents the size of that difference. Depending on the platform, the visual may be labeled differently, but the underlying concept is the same: spread = bid minus ask difference (in absolute terms, ask minus bid).
Example and checks
Suppose a quote shows bid = 1.2500 and ask = 1.2502 for a currency pair. The bid-ask spread is 1.2502 − 1.2500 = 0.0002. If the quote later changes to bid = 1.2500 and ask = 1.2503, the spread becomes 0.0003, meaning the “spread line” (the displayed spread size) has widened.
A practical independent check is to compare the numbers you see: if the platform shows bid and ask, you can calculate the spread directly. If the platform also shows a spread value, it should match the computed difference, within formatting/rounding rules.
Relevant limitations and uncertainty
Spreads are not fixed. They can change as market liquidity and trading conditions change, including during high activity, volatile moments, or outside regular liquidity periods. A wider or narrower bid-ask spread can affect transaction costs, but it does not guarantee that any future price movement or trading result will be better or worse.
Because different providers may display quotes with different formatting or rounding, always base your understanding on the bid and ask relationship and treat any on-screen “spread line” as an indicator of that relationship rather than a predictive signal.