i should also mention that the glottis has nothing to do with the throat theory at all, and the choana only plays a small part, what you are actually looking at should be the curtain and the choana only and from there making your gradings. The reason i say the choana only plays a small part as many things can affect its opening. Take this for example Ronnie Bigwood (Another Throat specialist Nut) came to my loft and graded my best breeding hen, and he graded her at an 8 throat due to her choana slit wasnt open, I told Ronnie to come back in 2 weeks, I then treated my pigeons for ornithoses and mycroplasmosis, the result was on the next viewing her choana was open to the right amount resulting in her grading to go higher. The moral of the story is that for a true grading, your birds also need to be 100% healthy, and when viewing they must be stress free.