Profile Tolerances: Line and Surface, and Their Application
The universal control
Profile of a line controls each cross-sectional element independently between two parallel lines offset from the true profile. Profile of a surface controls the entire surface between two surfaces offset from the true profile. Because no other control combines size, form, orientation, and location, profile works on curved, complex, or irregular surfaces where position is impractical - and even for coplanarity of interrupted faces.
Defaults matter. A profile zone is bilateral and equally disposed about the true profile unless the unequally disposed symbol U (2018) or an offset phantom line in older practice is shown. With U, the number following the symbol is the portion of the zone outside the true profile: a surface profile of 0.4 with U 0.1 gives 0.1 outside the true profile and 0.3 inside.
Profile with datums locates the surface. Profile without datums controls only form and size relative to itself, useful for free-form surfaces whose location is unimportant. The all-around symbol extends the zone around an entire cross-section; the all-over symbol applies it in all three dimensions.
Traps: forgetting that profile requires basic dimensions to locate; assuming profile of a line controls the surface between element lines (it does not - a part can pass every line check and still fail surface profile); and confusing a composite profile's upper segment, which locates to datums, with a lower segment, which controls only form and orientation.