The Trefoil

Near Los Alamos, New Mexico

March 10, 2020

The trefoil (the name of the sign above) should not be confused with UVa’s men’s basketball team’s “tresfoil” defense.

In 1946, the ionising radiation trefoil started as a ‘doodle’ produced by a group of people at the Berkeley Radiation Laboratory, University of California, who were keen to develop a warning symbol for this new kind of dangerous material.

The trefoil represents the activity of an atom.It was first rendered as magenta on a blue background. However, the blue proved to be unpopular and, in 1948, Bill Ray and George Warlick, at the Oak Ridge National Laboratory in Tennessee, cut out the magenta symbol and stapled them on to cards of different colours. To get a sense of what stood out the most they put the cards outdoors and found that, from a distance, magenta on yellow best conveyed the idea of danger, which became the standard design in early 1948.

The trefoil is black in the international version, which is also used in America.The symbol is drawn with a central circle of radius R, an internal radius of 1.5R and an external radius of 5R for the blades, which are separated from each other by 60°.

The sign is commonly referred to as a radioactivity warning sign, but it is actually a warning sign of ionising radiation.

It is still an open question as to whether the trefoil is historic or historical. 🤪

11 views1 comment

Welcome to my webpage.  I'm on a journey across the USA to visit all 22 Paris' - and points in between.  I'll be sharing thoughts, photos and videos along the way - as I search for answers to questions that bother me so.


Read More


About Me

© 2023 by Going Places. Proudly created with