Cross rate
The rate of a pair without the dollar, such as EUR/GBP. It can be derived from two pairs that include the dollar. A broker quotes the cross directly, but the spread on such pairs is often wider.
Example
EUR/USD is 1.10 and GBP/USD is 1.30. Divide: 1.10 / 1.30 ≈ 0.846. That is EUR/GBP.
What not to do
Do not trade a cross without checking which two pairs it is built from: a move in the dollar affects both legs. And compare the spread with the major pairs.
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