Formula 1 drivers are now permitted one change of helmet design, and for one grand prix only, during the course of a season.
Prior to the start of the 2015 F1 campaign the FIA introduced a new regulation designed to prevent drivers from continually changing their helmet colours, as the likes of Sebastian Vettel often chose to do.
It was felt forcing drivers to retain predominantly the same helmet throughout a season would ensure they were more recognisable to the fans.
Article 9.1 of this year's sporting regulations states: "In order for drivers to be easily distinguished from one another whilst they are on the track, the crash helmet of each driver must be presented in substantially the same livery at every event during a championship season."
The word 'substantially' was introduced to allow the potential introduction of new sponsors, or other minor changes.
There has since been a slight relaxation of the rules and for one race only per season a driver can run with a different design.
Once it has been used, the driver then has to revert back to the design with which they started the season.
For this weekend's Monaco Grand Prix, Williams drivers Felipe Massa and Valtteri Bottas are both using different-liveried helmets.
"They [the FIA] have allowed this," said Massa.
"They have allowed the drivers to use once per year a different helmet, which is something we have been asking for the whole season.
"Maybe what Sebastian was doing was too much, but if you want to do something special, once a year is nice and they [the FIA] accept that."
Describing his own new design for the Monaco race, Massa added: "It has been done by a very famous Brazilian artist [Os Gemeos].
"They do graffiti, they paint on the road. They have become really, really famous in the whole world, so they have painted the helmet for me.
"I have also my son and wife here, so it is a very special art that I will keep."
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