The definition of a  cross pair is the creation by two separate currency pairs with USD and Non-USD employed as the basis to calculate. The term is triangulation and done through the various legs. What is created is a cross pair. Across pair triangulation and creation of a currency pair is the opposite to a synthetic currency.

A synthetic currency is created by pairing a convertible currency to a blocked currency. A blocked currency lacks ability to freely trade and trades on the black market. Argentina ARS and other South and Central American nations are prime examples. The last manner to create a synthetic currency is to match a currency pair to a non-Deliverable Forward. Many nations Forwards are non-Deliverable. Prime examples include Brazil, South Africa, South Korea, Israel.  The key is to match the spot equivalent currency to the NDF to lock in a price on X date and  to guard against volatility.

A few examples to triangulate cross pairs include JPY. EUR/JPY is created by EUR/USD X USD/JPY. GBP/JPY is created byGBP/USD X USD/JPY.  NZD/JPY is created by NZD/USD X USD/JPY.

CHF/JPY is created by USD/JPY Divided by USD/CHF.  CAD/JPY is created by USD/JPY divided by USD/CAD.

GBP/NZD is created by GBP/USD divided by NZD/USD. GBP/CAD is created by GBP/USD X USD/CAD. AUD/EUR is created by dividing AUD/USD by EUR/USD. EUR/GBP is created by EUR/USD divide by GBP/USD. AUD/NZD is created by AUD/USD divided by NZD/USD.

A pair like MXN/RUB triangulates as USD/RUB divide USD/MXN but sometimes must factor USD/MXN, EUR/RUB, and EUR/USD. Depends on the time of day, liquidity and spreads. When those markets are open and traded, spreads are low but widen significantly upon market closes. Both RUB and MXN are oil producer currencies however  RUB tracks Brent while MXN follows WTI. Both respond to Brent/ WTI spreads. OPEC influences more Brent than WTI. Overall 70% MXN exports travel to the United States, therefore  MXN is most sensitive to DXY.

Cross pairs move more than underlying USD and Non but are never as liquid. Liquidity depends on USD V Non-USD for cross pair movement because those pairs are most liquid.

One factor influences RUB is the CBR cut its USD reserves below Euro for the first time since 2008. USD reserves since Jan 1 was 39.6% from 44.8% in 2015. Euro current is 46.1% V 41.5%.

The general rule for cross pair triangulation is if USD is the base for both currency pairs then divide to obtain the cross pair price.  If USD is located in the quote position then divide, otherwise multiply. Basic formula is A/B X B/C = CB. Cross rates equal the ratio of the two corresponding pairs.