The price of crude oil has fluctuated sharply since the first oil crisis exactly thirty years ago, plunging the world economy into phases of boom and bust.
Opec's decisions to cut output in 1973 led to a global recession
Oil is the lifeblood of industrial economies.
Some would say the technology exists to break that dependence. But it is still not very widely used.
Transport is the sector most dependent on oil.
Petroleum powers almost all cars, lorries, buses and planes and many trains too.
Well over 90% of the energy for transport is produced by burning oil products.
Crude oil is also an important source of energy for electricity (though not the largest).
It is also a vital raw material for plastics, fertilisers and many other products of the chemical industry.
So when the taps are turned off, or when it just gets expensive, the economic consequences are severe.
First oil crisis
It was during the oil crisis of 1973 when the world suddenly realised just how dependent it was on the Middle East and OPEC for its oil.
In the wake of the Yom Kippur war between Israel and its Arab neighbours, the Arab oil producing countries slapped an embargo on supplies to the US on 20 October 1973.
For the wider world, oil prices went through the roof, from around $3 a barrel before the war to over $11 by early the following January.
It gave economics a new word: stagflation.
High oil prices contributed to inflation. They also imposed higher costs on consumers and businesses, slowing economic growth in the process.
Not long after the crisis there was a recession in 1975, the first of four world downturns where oil price increases caused by events in the Middle East played a key role.
There were often delays before the price rise did its wider economic damage, but the link was still there.
In 1973 it was the Yom Kippur war and the Arab oil embargo that led to a global recession in 1975.
Then at the end of the decade there was Iran's Islamic Revolution.
Higher fuel costs in developing countries hurt growth
Production in Iran dived from a peak of around six million barrels a day shortly before the revolution, to a low of below one million.
And just as oil output started to recover in the early 1980s, Iran became embroiled in a long and nasty war with Iraq.
The resultant supply disruptions contributed to another global recession in 1982/3.
Gulf War I and II
And then there was the Gulf crisis of 1990 and 1991.
The Iraqi invasion of Kuwait and the subsequent sanctions cut off oil supplies from the two countries.
Again, the price of crude oil climbed and there was another global recession.
And then there was 2001.
That is slightly harder to read.
It was a year of weaker global performance, arguably a recession (the IMF says not, but plenty of other economists say it was).
And that was preceded by a period in which oil prices were on the high side.
In real terms (allowing for general inflation), although nowhere near the peaks of the mid-1980s, oil prices were still sharply higher than the lows they hit in 1998.
This time Middle East political and military developments did not play as much of a role.
It was more a case of the producers' cartel restraining supplies at a time when demand was growing strongly.
Perhaps the price rise was a factor in the 2001 slowdown - not the main one, which was the bursting of the technology share price bubble, but a significant one nonetheless.
1973 was the first taste of all this, and it has to be said for oil consumers it has left a nasty after-taste.