@Christopher2222 . You need the typset because otherwise the degree symbol is spaced wrongly as in http://www.mapleprimes.com/questions/207121-How-To-Display-Degrees--In-Odeplot

plot(sin((1/180)*Pi*x), x = -180 .. 180, tickmarks = [[seq(180*i*(1/4) = typeset((180*i*(1/4)),`°`), i = -4 .. 4)], default])

You can use quotes and the expression pallette. Also HTML entities 

plot(arcsin(x), x = -1 .. 1, axis[2] = [tickmarks = piticks]); plot(180*arcsin(x)/Pi, x = -1 .. 1, axis[2] = [tickmarks = [90 = typeset(90, "°")]])

Not all of them: ☠ is not available.  Thanmks for your answer.

@Christopher2222 On my system F5 toggles to 1D math mode not text mode like ctrl-T.

@Carl Love  Sorry I should have said that. It is Maple 2016.0  running on Windows 10 (64 bit). It is up to date.

I deleted the paragraph that was showing this behaviour and started again. The effect stopped and I cannot reproduce it. Since I am a new user there would have been a lot of hitting the wrong keys so maybe I chanced upon a bug.

-- Peter

@Carl Love  Yes - I can have award the trophy. Good

@Christopher2222 usually I jiust click the like button instead of filling the forum with a Thanks!, Me too!! +1 and the like. But I presently don't have enough reputation to upvote an answer to my question. There does not seem to be a way to marek an answer as accepted either.