Not all overhangs need supports - the 45 degrees rule
Not all overhangs need to be supported. The general rule of thumb is: if an overhang tilts at an angle less than 45 degrees from the vertical, then you may be able to print that overhang without using 3D printing support structures.
3D printers use a very small horizontal offset (barely noticeable) between consecutive layers. So, a layer does not stack perfectly over the previous layer but rather stacks with a tiny offset. This allows the printer to print overhangs that do not tilt too much from the vertical. Anything below 45 degrees can be supported by the previous layers, anything north of that figure, not. Forty-five degrees is considered to be the line of failure.
This aspect is best illustrated with the letters Y and T. The two overhangs in the letter Y have an angle less than 45 degrees with respect to the vertical. Therefore, if you want to print the letter Y, you can get away without using any 3D printing support structures.
On the other hand, the overhangs in the letter T have an angle of 90 degrees with the vertical. So you must use 3D printing support structures to print the letter T, otherwise, the result will be a mess, as illustrated below.
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