Main content
4th grade
Course: 4th grade > Unit 11
Lesson 2: Angle introductionNaming angles
CCSS.Math:
Sal names angles based on their vertex and endpoints. Created by Sal Khan.
Want to join the conversation?
 Couldn't we distinguish two angles  one "interior" and another "exterior"?(10 votes)
 When you write CAP or XYZ or whatever, why is it always capital?(5 votes)
 When naming an angle or a shape, mathematicians tend to use capital letters, because they aren't expecting you to do math operations with those letters. They are known values and don't need to be calculated. They are like names, as opposed to other kinds of words. If someone were to write your name as "abirbakshi" instead of "AbirBakshi," then someone who didn't know it was your name might think it was a regular word with a defintion (or value) and try to measure or describe you like they would a table or other object. That would be wrong. So, we name things that are known values with capitals to differentiate between them and the small letters that we use to represent unknown values as used in algebra or geometry. It's okay to measure or calculate using "x" or "y" but not "X" or "Y."(8 votes)
 How did you get tht answer(7 votes)
 what does m<1 or m<2 mean? (< is a substitution for the angle symbol.)(2 votes)
 The "m" stands for "measure" or "the measure of." Therefore, m<1 means "the measure of angle one," and m<2 means "the measure of angle two."(10 votes)
 Hi I am on the updated course and am struggling with NAME ANGLES. If anybody has finished ths course could you please explain? Thanks.(5 votes)
 Angles need to be expressed with names just like humans. The way we name them is by using the points given. For example, image an open angle with point A on the first line and point B as the vertex (the point where the two lines intersect to form the angle) and point C on the other side. It's something like this:

*A (imagine the dot is on that line)

 (imagine C is on this line lying down)
*B _______*C_______
(imagine B is where they touch each other)
If you can't visualize it then draw it on paper.
So the way we would call this angle is Angle ABC because the if you imagine drawing through the angle in one line you would start your line at point A and trace all the way to B, make a turn, and go to point C.
Do you understand the first part? If not you can ask me again for more explanation :D
Ok now for the second way. We can name an angle simply by saying *angle [B]* (B is the vertex).This only applies when the vertex *DOES NOT INVOLVE IN OTHER ANGLES!* The graph above would be an example because the vertex B only form that one angle. BUT IN THIS EXAMPLE:
For example: Imagine two lines crossing each other, and B is the point where they cross.
the vertex point B forms a lot of angles. Therefore you cannot name the angle with one letter because people won't know which angle you are talking about. In this situation you need to use three letters method.(3 votes)
 At0:49, Sal said that you can't name the angle <CPA <PAC, why?
I know he explained, and probably properly, but I still don't understand! This is frustrating!
...
Sorry for the outburst. : Someone, please explain!
This problem has been resolved. Thank you!:)
(3 votes) The letter in the middle of the angle’s name has to be the point of the angle’s vertex. We know from the name ∠CPA that point P is the vertex of the angle in question. Since P is the vertex, any other name we could give to that same angle would have to stay true to which point is the angle’s vertex, meaning that P would have to be in the middle of the angle’s name. Another acceptable name for ∠CPA would be ∠APC
∠PAC would mean that point A is the angle’s vertex, which would mean that ∠PAC and ∠CPA couldn’t be naming the same angle.
Does that make sense?(6 votes)
 Is there a practice quiz after this?(3 votes)
 You will have several opportunities to practice this with quizzes and tests, so remember those names and what they stand for.(5 votes)
 how are some angels different from other angels(2 votes)
 Some angle are different from others because some might be big and others might be small.(4 votes)
 which type of angel is 180?(2 votes)
 My memory from math class is that the order does matter because otherwise it's ambiguous which angle you're referring to. Every angle can be measured as either x or 360x. The convention is to name it counterclockwise. APC would be the small angle and CPA would be the large angle.(2 votes)
 I've never heard that before. They both refer to the same angle unless reflex angle is specified.(1 vote)
Video transcript
What is another
name for angle CPA? So we have C, we have
P, and we have A. So angle CPA is going to be
this angle right over here. So what's another name for it? Well, I could start off with A,
and I could call it angle APC. So this is the same
thing as angle APC. Now is that any
one of my choices? Yeah, that's this
choice right over here. If someone said angle P,
that wouldn't give you a lot of information. That kind of says, well, that's
only giving me one point. I don't know, maybe
it's referring to that angle, that angle,
that angle, that angle. So that's really not giving
me enough information to specify this angle
right over here. Angle PAC would be an angle
formed if you actually had a line here, if you
had a line between AC. Angle PAC would be
this angle right over here, which is
clearly a different angle. Angle FPA would be
this angle right over here, which is also
clearly a different angle. So the only choice that
makes sense is angle APC.