EU and Switzerland. The scientific calculators HP 39gII and HP Prime support the unit symbol "tr" for turns since 2011 and 2013, respectively. Support for "tr" was also added to newRPL for the HP 50g in 2016, and for the hp 39g+, HP 49g+, HP 39gs, and HP 40gs in 2017. An angular mode TURN was suggested for the WP 43S as well, but the calculator instead implements "MUL" (multiples of ) as mode and unit since 2019. Unit conversion One turn is equal to (≈ ) radians, 360 degrees, or 400 gradians. Proposals for a single letter to represent 2π In 1746, Leonard Euler first used the Greek letter pi to represent the circumference divided by the radius of a circle (i.e., = 6.28...). In 2001, Robert Palais proposed using the number of radians in a turn as the fundamental circle constant instead of , which amounts to the number of radians in half a turn, in order to make mathematics simpler and more intuitive. His proposal used a "π with three legs" symbol to denote the constant (). In 2008, Thomas Colignatus proposed the uppercase Greek letter theta, Θ, to represent 2. The Greek letter theta derives from the Phoenician and Hebrew letter teth, 𐤈 or ט, and it has been observed that the older version of the symbol, which means wheel, resembles a wheel with four spokes. It has also been proposed to use the wheel symbol, teth, to represent the quantity 2, and more recently a connection has been made among other ancient cultures on the existence of a wheel, sun, circle, or disk symbol—i.e. other variations of teth—as representation for 2. In 2010, Michael Hartl proposed to use the Greek letter tau to represent the circle constant: . He offered two reasons. First, is the number of radians in one turn, which allows fractions of a turn to be expressed more directly: for instance, a turn would be represented as rad instead of rad. Second, visually resembles , whose association with the circle constant is unavoidable. Hartl's Tau Manifesto gives many examples of formulas that are asserted to be clearer where is used instead of , such as a tighter association with the geometry of Euler's identity using instead of . Initially, neither of these proposals received widespread acceptance by the mathematical and scientific communities. However, the use of has become more widespread, for example: