🚀 go-pugleaf

I play with eletronics as a hobby, I'm self taught (I know there's alot I don't know, but I'm not illiterate on the subject), and have never taken a formal course or class on the subject. <This is my excuse>

Anyway, I'm building some simple transmitters, from schematics.  Anyway, it threw me a curve.  I have never played with crystals, and there is something I don't understand.  On the schematic it says:

"X1 - 52 Mhz, any 3'rd Overtone type"

And on a similar one it says:

"X1 - 16 Mhz, Fundamental"

What exactly is 3'rd Overtone type, and Fundamental?  I know what an overtone is, but I'm not quite sure it's related here.  I poured over by books, most of them don't disscuss crystals at all.

Can anyone help me out, and further my knowledge?

             Thank You In Advance.
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