Stack and Reach
These are numbers that you won't measure on your existing bike, but if your bike is new enough, you can probably look them up easily. A professional bicycle fitter, or custom builder will often use these numbers when working with you. If you are working with a fitter that uses reach and stack, then these numbers will be useful to you when selecting a size.
Reach is the horizontal distance from the center of the bottom bracket/crank to the top middle point of the head tube.
(Not to be confused with 'reach to the handle bar' measurement). This method of measuring the top tube is great because it takes away a manufacturer's ability to
'cheat' when listing their top tube measurement on small bicycles while using really steep seat tube angles.
Stack is the vertical distance, in centimeters, from the center of the frame’s bottom bracket/crank to the top middle point of the head tube (where the fork passes through the frame).
Reach and Stack are numbers that help standardize the fit geometry for bicycle comparisons, especially for the reach. For instance, knowing the reach of a bicycle (as measured by this definition) will indicate whether or not the manufacturer is trying to use
trickery to make their top tube seem shorter. I've written a lot on this subject if
you dare to read.
