Note: There are diagrams I can't post here, it may be worth clicking the link.
Russian President Vladimir Putin and his Chinese counterpart Xi Jinping have praised the deep ties between their countries, during a meeting in Beijing.
It was their fourth meeting since Russia invaded Ukraine in February 2022.
In that time, Beijing has become a vital partner for Moscow, as it seeks to soften the impact of sanctions imposed by the US and other countries.
Is China providing Russia with weapons?
China has repeatedly denied allegations that it supplies Russia with weapons.
In an interview with BBC News, US Secretary of State Antony Blinken said: "What's not happening is the provision of actual arms by China to Russia for use in Ukraine."
However, China has been accused of building up Moscow's war machine by providing critical components.
Mr Blinken said: "Those are being used to help Russia on what's an extraordinary crash course effort to make more munitions, tanks, armoured vehicles, missiles."
About 70% of the machine tools and 90% of the microelectronics Russia imports come from China, he added.
Sanctions announced by Washington in May targeted about 20 firms based in China and Hong Kong. It said one exported components for drones, while others helped Moscow bypass Western sanctions on other technologies.
China defends its trade with Moscow by saying it is not selling lethal arms and "prudently handles the export of dual-use items in accordance with laws and regulations".
Beijing exports more than $300m worth of dual-use items - those with both commercial and military applications - to Russia every month, according to an analysis of Chinese customs data by the Carnegie Endowment think tank.
It says the list includes what the US has designated as "high priority" items, which are necessary for making weapons, from drones to tanks.
RUSI, a UK-based think tank has also cautioned about the potential use of Chinese satellite technology for intelligence on Ukraine's front line.
How much has trade between China and Russia increased?
Beijing has become Moscow's key supplier of cars, clothing, raw materials and many other products, after Western countries imposed sanctions on Russia.
Trade between China and Russia reached a record $240bn (£191bn) in 2023, up more than 64% since 2021 - before Russia's invasion of Ukraine - according to official figures from China.
Russian imports from China were $111bn and its exports to China $129bn, the figures show.
At their meeting in Beijing in May, Mr Xi and Mr Putin praised growing trade between the two countries.
They highlighted that the two nations now use their own currencies for 90% of trade, instead of US dollars.
Mr Putin also said he welcomes Chinese carmakers in Russia. This came just days after the US announced a quadrupling of tariffs on China's electric vehicles to 100%.
The export of Chinese cars and parts to Russia reached $23bn in 2023 - up from $6bn the previous year.
"Russian natural gas is fuelling numerous Chinese households, and Chinese-made automobiles are running on Russian roads," said China's Foreign Minister Wang Yi in March.
However, some experts consider this a "lopsided" relationship in which Russia is more dependent on China than vice versa.
As of 2023, China has become Russia's top trade partner, while Russia is China's sixth-largest trade partner.
How much oil and gas does China buy from Russia?
Almost half of all the Russian government's annual revenues come from oil and gas.
Its sales to the US, UK and EU countries have plummeted since the invasion, because of sanctions.
A significant amount of this shortfall has been made up with increased sales to Asia - in particular, China and India.
In 2023, Russia surpassed Saudi Arabia to become China's top crude oil supplier. Beijing imported 107 million tonnes of crude oil from Moscow - a 24% increase from 2022.
The G7 group of "advanced" economies, along with the European Union and Australia, has also tried to limit Russia earnings by imposing a worldwide cap on the price of its oil transported by sea.
However, China has continued to buy Russian crude at above the price of the cap.
India, which has continued to maintain its decades-old relationship with Russia, has also been a major buyer of its discounted oil since the invasion.
Russia's share of India's total oil imports hit a record high of 44% in June 2023, according to the Bank of Baroda, an Indian state-controlled lender.
In 2023, China also imported eight million tonnes of liquefied petroleum gas (LPG) from Russia, a 77% increase from 2021.
The two countries also plan to expand energy ties, including a new pipeline - called the Power of Siberia 2 - to export natural gas from Russia's western Siberia region to north-eastern China.
China already receives gas from Russia through the original Power of Siberia pipeline, which has been in use since 2019.
China was briefly an observer state to Warsaw Pact and then pretty quickly completely withdrew.
After sino-sovet split in the early 60s Sino-soviet relations did not really normalize until Gorbachev.