# When to use alpha vs theta for angles

1. Dec 27, 2011

### PhysEd

Pretty simple question.

When we draw triangles or other pictures with angles, when do we mark the angles as alpha and when do we mark them as theta?
Is there some kind of convention or is it just whatever is free in the equation?

It makes sense to me to use alpha for the first angle then beta for the second and so on.
But how did theta get involved?

Any insight is appreciated.

2. Dec 27, 2011

### rollcast

Theta is often used in examples involving trigonometry.

Theta is usually the unknown angle which you will be trying to find.

3. Dec 27, 2011

### quasar987

It is an unwritten convention that theta, alpha, beta, phi, psi, gamma are used to denote an angle, just like x,y,z are traditionally used as variables in an equation, but in principle, you could use whatever symbol you like. I don't know why these are the symbols commonly used for angles but it's like this. Perhaps it goes back to Euclid? However, if your goal is to be understood, then stick with theta, alpha, beta, phi, psi. Just like you wouldn't want to say "consider the equations axy - zc³ + bx² where a,b,c are variables and x,y,z are constant. That would just be confusing.

4. Dec 27, 2011

### chrisbaird

Usually the Greek letters near the end of the alphabet, θ, ψ, φ, are used for angles that are variable in the problem, and the Greek letters near the beginning of the alphabet, α, β, γ, are used for angles that are constant in a problem. This mirrors how Latin letters near the end of the alphabet, x, y, z, are used for variable coordinates and those near the beginning of the alphabet, a, b, c, are used for constants. This is, of course, a convention - but a very logical convention.

5. Dec 27, 2011

### calculusian

In physics some angles have definate symbols.For eg
FRICTION-
Angle of friction-theta
Angle of repose-alpha
correct me if i am wrong.......

6. Dec 27, 2011

### PhysEd

Thanks everyone for the answers, I compared all your notes to the physics course I am following and I think chrisbaird solved my confusion.

The professor uses theta when the angle is unknown and alpha/beta when they are known!

Awesome, thanks again.

7. Sep 8, 2013

### rwlangbe

I wanted to add theta (θ) is used inside a circle for the angle relative to the horizontal because if you look at the theta symbol closely, that is what is looks like. A circle with a horizontal line across it. Phi (φ) is used for the angle relative to the vertical because the symbol looks like a circle with a vertical line. Alpha, Beta, Gamma are used because they are identified with A, B, C. The three sides of the triangle.