Cross currencies refer to currency pairs that do not involve the US dollar (USD) as one of the currencies. In forex trading, the vast majority of transactions involve the USD as one of the currencies, making it the world's primary reserve currency. However, cross currency pairs allow traders to directly exchange two different currencies without converting them into USD first.

For example, EUR/GBP (Euro/British Pound) and AUD/JPY (Australian Dollar/Japanese Yen) are cross currency pairs. When trading these pairs, traders are speculating on the relative strength or weakness of one currency against the other, without the influence of the USD.

Cross currency trading can be advantageous for several reasons. It allows traders to diversify their currency exposure and hedge against currency risk more effectively. Additionally, it provides opportunities for profit by exploiting the differences in interest rates and economic performance between two countries.

However, cross currency trading also presents some challenges. Liquidity may be lower compared to major currency pairs, leading to wider spreads and potentially higher trading costs. Additionally, because cross currency pairs are less actively traded, they may be more susceptible to market volatility and sudden price fluctuations.

Overall, cross currencies play a significant role in the forex market, offering traders opportunities for diversification and profit, albeit with some unique considerations compared to trading major currency pairs.