If you are one of millions of Londoners who stoically battle through the Tube's rush-hour commute, you will no doubt pass dozens of Transport for London signs and notices on your journey.
But how much attention do you actually pay to them?
Has their familiarity as part of everyday visual clutter led to them becoming almost invisible, losing all meaning beyond shape and colour?
Take the following examples:
"No eye contact. Penalty £200."
"We apologise for any incontinence caused during these engineering works."
"Peak hours may necessitate you let other people sit on your lap."
Look familiar? Perhaps not. Yet these are a few of a growing number of guerrilla stickers that have recently appeared on the network.
They use the same fonts and designs as London Underground's famous branding.
But they subvert the intended message making often amusing but sometimes serious points about anything from overcrowding to Tube etiquette.
Some commuters are amused by the stickers, including London blogger, Annie Mole, who says: "A number of them are funny and it breaks up the journey a bit."
But British Transport Police (BTP) warned: "The costs of graffiti are substantial for the railway industry in terms of repairs and clean-up, and can leave permanent scars on the infrastructure."
Transport for London was approached by BBC News but declined the opportunity to comment.
The BBC spoke to a spokesman for the website which sells stickers similar to some of those which have appeared on the Tube.
He referred to himself as James, from east London, and said his site had sold about 200 stickers "for the Underground" so far this year, at an average cost of £2 per sticker.
He believes the stickers are about "taking back power".
"It's a form of rebellion, whether it be due to the current climate of doom and gloom and people wanting to brighten their day," he said.
"It's almost as though people are treating you as a drone and the signs are very serious," James added. "This is a bit of escapism and freedom that people can express relatively easily."
He said he has seen the stickers growing in popularity.
"I have seen more signs, more stickers and other designs, particularly based on the more common signs you see on the Underground," said James.
"Stickers are becoming more popular recently because people are able to produce their own.
"People can just take the sticker out, stick it off and be gone in a couple of seconds."
But he defends his website for selling the stickers.
"I'm not putting them up and the website cannot endorse them being stuck on the Tube," he said.
"It's not graffiti. Stickers can be removed," he added. "It's up to people where they stick the stickers. I don't think it's been doing any particular harm."
Annie Mole, who has been writing the award winning Going Underground blog for the past 10 years, agrees there has been an increased number of stickers appearing.
"Over the years that I've been blogging, I've seen more and more inventive stickers," she said.
"The fact that they're now commercial to a small extent definitely shows there is a growth or an interest in the stickers."
She says when the stickers started appearing, they were "a bit subversive" but more recently "people have been thinking how far can they really go with them".
But she says she thinks the stickers are "harmless".
"It's less destructive to me than graffiti, where people put their own mark on property," she said.
BTP said graffiti was "unwanted vandalism that causes criminal damage" and "will not be tolerated".
"It is a blight on our society and becomes an eyesore for many residents who overlook the railway," a BTP spokesman added.