When both parts have plain clearance holes and the standoff has to supply the threads.

Nuts at each end do the clamping, so neither part needs to be tapped.

It also suits a tapped hole at one end and a nut at the other, which is common in chassis work.

Where one part is tapped and the other has a clearance hole, a male-female standoff is usually neater.

Where both parts have clearance holes and nothing should project, female-female with two screws is the alternative.

The choice is really about which parts already have threads and which do not.

Because the standoff has to be held while nuts are tightened, and there is no other way to react the torque.

With a nut at each end, turning either one turns the standoff unless it is gripped.

Flats give a spanner something to hold, which is not optional in this type as it is in others.

Knurled round bodies exist but give much less grip against real tightening torque.

Where access to the body is restricted, tightening becomes difficult and a different standoff type may suit better.

In polymer versions the flats round off readily, so torque has to be modest.

Body length plus both thread lengths - three dimensions rather than two.

Body length sets the gap between the parts, which is normally what the design states.

Each thread length must pass through its own part thickness and leave enough for the nut to engage fully.

The two ends are frequently different lengths, because the two parts are different thicknesses.

Overall length is sometimes quoted instead, so which figure a catalogue means is worth confirming.

Ordering on body length alone gives the right gap and no way to secure it.

Yes - adaptor standoffs with different threads at each end are a standard product.

They let a component with one thread size mount to a chassis with another without modifying either.

Mixed metric and imperial adaptors exist for the same reason, and are useful where equipment from different markets is combined.

Marking or measuring is important, since a mixed-thread standoff looks like a single-thread one.

Orientation matters for these, unlike a symmetrical male-male standoff.

Where several are used in one assembly, a fitting order or a colour mark prevents them going in reversed.

They are vulnerable during handling and service - bent studs and damaged threads are the common complaint.

When the assembly is apart, both ends project with nothing protecting them.

A bent stud will not enter its hole, and a damaged thread will not take its nut, so either stops reassembly.

Thread protectors or caps are worth using where units are transported dismantled.

For assemblies opened frequently, female-female standoffs avoid the problem entirely.

A thread chaser or die is the practical repair for a lightly damaged end.

Nuts matching the thread and the material, with a locking type where vibration is expected.

Plain nuts are adequate for static assemblies where the joint is properly tightened.

Nylon-insert or all-metal locking nuts are used where the equipment vibrates, since there is no other locking feature.

Washers spread load on soft panels and prevent the nut marking a finished surface.

Material compatibility matters, since the nut, the standoff and the panel are all in contact.

In polymer standoffs, over-tightening a nut strips the stud thread long before the nut fails.

Light panel and bracket loads - the thread and the slender body are both limits, and side loading is the harder case.

Tension along the axis is resisted by the thread engagement at each end.

Side loads put the body into bending, which slender standoffs resist poorly, particularly in polymer.

Longer standoffs are proportionally weaker in bending, so a long slender pillar is the least capable arrangement.

Using more standoffs spreads load better than using fewer larger ones.

For anything carrying real weight or impact, a bracket or a structural pillar is the correct part.