This shows the proper vent location, minimum length of trap arm, etc.
Basically, the trap arm must be a minimum of two pipe diameters in length before reaching the vent. Note, the first image here shows the trap butting up against the sanitary tee and it is not actually 2 pipe diameters away in the drawing.
To quote PBwrencher from plbg.com
A "Crown Vent" is a vent that is located within 2 pipe diameters of the trap it is serving. When a vent is that close it is more subjected to fouling of hair, soap oils, grease, and other debris because; the flow of water does not have a chance to dissipate thus reaching the top of the pipe where the vent is during discharge causing premature failure.
Thanks to Frodo on plumbingforums.com for pointing this out to me. Second image was supposed to be Frodo’s correction of the diagram.
Third image is my revision to remove the crown vent.