Even Vs Odd
When we look at intervals, we see on a music staff that all notes have a line through the middle or not.
When both notes of an interval have a line through or both do not, then the interval is an ODD interval.
When one note has a line through the middle of it and the other does not, then the interval is EVEN.
This is another tool to quickly asses distance between notes.
It may mean the difference in determining a 6th or 7th interval and actually getting it right.
Try to first look from note to note and recognize whether the interval spaces are ODD or EVEN.