Locate against a flat surface and resist rotation without being held.

A round spacer touches a flat surface along a line; a square one bears across a face.

In a channel, a square cut-out or a folded corner, the flat sides position the spacer and stop it turning.

That means the assembly is located by the spacer rather than only by the fastener.

It also gives a larger, more stable bearing area against another flat component.

Where nothing needs locating, a round spacer is cheaper and more widely available.

Clearance must be checked on the diagonal, which is substantially larger than the nominal side dimension.

A square section's corners reach considerably further from the centre than its faces.

In dense assemblies the corners are what fouls a neighbouring component, a track or an enclosure wall.

Nominal size is quoted across the flat sides, so the diagonal is a calculation the catalogue will not do for you.

Where the fit is tight, a round spacer of the same bore occupies the least space.

Corner radii vary between manufacturers, which affects both clearance and how the spacer seats into a formed corner.

No - the range of sizes, lengths and materials is generally narrower.

Round and hex sections dominate the market because they cover most applications.

That means longer lead times and fewer stock lengths for square, which is worth establishing before the design commits to them.

Many designs that specify square spacers could use a round spacer with a separate locating feature at lower cost.

Where a square section is genuinely required, checking availability early avoids a redesign late.

Machining square spacers from bar is straightforward for small quantities.

For a given nominal size they present more bearing area against a flat surface, which helps in soft materials.

The spacer itself takes compressive load from the fastener being tightened, and how much it can take depends on material and section.

What changes with a square section is the area at the ends bearing on the parts, which matters when those parts are soft.

In stiff metal assemblies the difference is rarely significant.

None of this makes the spacer part of the joint's strength - the fastener still does the clamping.

Where high compressive load is expected, a metal spacer rather than a polymer one is the more important decision.

Usually, provided the clearance on the diagonal is acceptable - but a hex is easier to grip with a tool.

Both resist rotation once seated, so for anti-rotation alone either works.

A hex section matches standard spanner sizes; a square section generally does not.

A square spacer locates better against flat surfaces, which is where it earns its place.

Hex is much more widely stocked, so substituting the other way is easier.

If the reason for a non-round section is handling rather than location, hex is the more practical choice.

The same range as other spacers, though with a narrower selection in practice - metals for load and conduction, polymers for isolation.

Aluminium and brass are the common machined metals; stainless is available where the environment requires it.

Nylon and other polymers give electrical isolation and low weight, and moulded square spacers are common in electronics and enclosure work.

Glass-filled polymers resist creep better where the joint must stay tight over time.

Metal spacers in a mixed-metal assembly can create a galvanic path, which a polymer spacer breaks.

Corner radii and surface finish vary more between manufacturers than for round sections.

Plain is the norm; a shoulder or a moulded step is a way of locating the spacer in a hole rather than against a face.

A plain bore simply lets the fastener pass through and clamp the stack.

Shouldered and stepped spacers register into a hole in one of the parts, which locates the assembly without relying on the fastener.

That is useful where a hole is oversized or where the spacer must be positioned before the fastener is fitted.

The shoulder diameter then becomes a dimension the mating hole has to match.

For most square spacer applications the flat faces already provide the location, and a plain bore is sufficient.