1. ## What is straight?

I want to define straight using the definition of having 180 degrees, but how do you explain 180 degrees?

Also what types of symmetry are considered line symmetry?

2. Originally Posted by loutja35
I want to define straight using the definition of having 180 degrees, but how do you explain 180 degrees?
Also what types of symmetry are considered line symmetry?
First a comment. Many who have worked in axiomatic geometry do not like using ‘degrees’ as a measure of angles.

An angle is the union of two rays with a common endpoint; as in $
\angle BAC = \overrightarrow {AB} \cup \overrightarrow {AC}$

Now a straight angle would be the case in which $C-A-B$.
$A,~B,~\&~C$ are collinear and $A$ is between $B~\&~C$.