For millions of Londoners, the words "tube strike" mean one thing - a very long, tiring journey to work.
In this city of more than eight million people, industrial action affecting the London Underground can often have a greater impact than in other capitals.
The reasons lie largely in the way London developed over the centuries, as well as how the network commonly known as "the Tube" has expanded since the first line opened in 1863.
According to Professor Tony Travers, an academic at the London School of Economics, who is also chair of research at the Centre for Cities, a think-tank: "One of the hidden problems of London on a day when there's a strike is that its road system is not one that was developed to allow massive throughput of vehicles.
"In many ways most of the streets are inherited from history, there are very few wide boulevards or avenues that you'd find in other big cities.
"As a result, the capacity of buses, cars and other forms of transport to make up for the closed underground is very limited."
Another factor is that on the edges of London, residential areas were established along the route of the underground, which stretches to the kinds of places served by regular trains in many capital cities.
This increases its importance for people travelling in to the centre.
How the world's oldest underground should meet the specific demands of the the 21st century is the question at the heart of the latest 24-hour strike.
The four trade unions involved are unhappy at conditions offered to drivers working on the new all-night Tube service, which is due to start on September 12.
Steve Hedley, assistant general secretary at the Rail, Maritime and Transport Workers union, told Al Jazeera: "We're all for the night tube, it's a really good idea - as long as it's staffed properly and staffed safely".
Hedley claims that the only way to make the new service work would be to recruit more drivers.
"They're asking people to cover more nights and weekends," he says, "more unsociable hours and people just are not having it."
Transport for London, which runs the underground, insist staff will not be working extra hours, and point out that they are recruiting 137 extra workers.
This week, the unions rejected a revised deal including a two percent average pay rise for all Tube employees and a $750 launch bonus.
Boris Johnson, the elected Mayor of London, has urged them to put the offer to their members.
Johnson has said he is not concerned about meeting the launch target of September 12, but says the all-night Tube must happen this year.
Speaking ahead of the latest walkout, the mayor said: "It is crazy, when you have the technology, not to put a 24-hour service in a 24-hour city."
That is a sentiment shared by many Londoners, but it is far from clear when that service will be up and running.
Source: Al Jazeera