What's the purpose of a balance shoe?

A home owner from Philadelphia, PA says:
What's the purpose of a balance shoe? I am trying to select new balance, I don't have a true original. The top seems to be a 19-002, but I don't know how to determine the correct shoe. How far is the shoe supposed to stick out from the balance? I am also purchasing sash guides, your 18-033 is probably the best fit. A balance I purchased a few years back from your retail store is numbered 2410 if that helps.
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Paul from SWISCO responded:
The shoes for this kind of balance help keep it stable in the sash. If it sticks out too far, the window will be difficult to open and close due to the excess friction. If it doesn't stick out far enough, it will rattle in the track and likely not work properly.

Without having the original balances on hand, it will be difficult to determine exactly what you need. The stamp number doesn't really offer a clue, since that only tells me how strong your spring is. Since you're using the 18-033, however, you may have had the Series 390 channel balance. This balance has been used in the same windows that also use the 18-033. However, please consider that I cannot guarantee that Series 390 is a match. Look it over and tell me what you think. Also see if you have an identical window in the house that uses the same balances.
A quick learner from Philadelphia says:
Please recommend balance length, window glass is 23' and 25' long. Both of the windows now have 25' balances number 24XX. I know this shouldn't be.
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Paul from SWISCO responded:
You say that your glass is 23" x 25". Which is the width and which is the height? Also make sure you're only measuring the visible glass. If the visible glass is 23" high, then I would recommend the S390-23.
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