The most common types are these:
Double 45's they look like a Tipee or upside down V. The sound characteristics are usually boxy and dry with a very controlled focused sound, they usually track fast strokes very well (which is why a number of companies such as Spaun that cater to punk and metal drummers us them) but don't sustain as long. A lot of guys prefer these, they are very plug and play, even if the drum is poorly tuned it usually sounds pretty good.
Round over edges, the are an upside down lopsided U shape. Very round and warm, not much sustain though, a lot of wood contacts the head so it tends to dry out overtones heavily. Jazz kits of the 30's and 40's used these edges and are sought after the warm fat tones they produce.
Single 45's, very open sounding with a lot of sustain, can ring too much with thinner heads and higher tunings. These are what your larger manufacturers use for the most part. You can get some great overtones if you want them, or you can tune them out, these tend to have the best tuning range and resonance. But tuning them can be an exercise in patience.
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