Because it can be held - with fingers or a spanner - while the fastener is started.

A round spacer spins freely, which is awkward in blind, overhead or one-handed assembly.

The flats also stop the spacer rotating against a surface it might mark during tightening.

They are easier to pick up and to distinguish from other parts on a bench, which speeds repetitive assembly.

Being machined from hex bar, they often hold better length tolerance than cut round tube.

In every other respect they behave identically to a round spacer of the same material.

No - a spacer has a plain bore, a standoff is threaded, and they are the most commonly confused pair in this branch.

Photographs of the two look nearly identical, since both are hexagonal sections with a hole down the middle.

A spacer needs a fastener passing all the way through the stack; a standoff accepts a screw at each end independently.

That changes how the assembly is built: standoffs allow layers to be added and removed one at a time.

The catalogue check is simply whether the bore is described as threaded, and at which ends.

Ordering one when the design assumed the other is a common and immediately obvious mistake at assembly.

Across flats for the tool, across corners for the clearance.

The across-flats dimension is what a spanner or socket engages and is the size usually quoted.

Across corners is larger, and it is what actually determines whether the spacer fits between adjacent components.

In dense assemblies the corners are what fouls a neighbouring part or a track on a board.

Bore, length and across-flats together define the part for ordering.

Where clearance is very tight, a round spacer of the same bore takes less space than a hex one.

Usually, because they are machined from hex bar rather than cut from tube.

Machining allows the ends to be faced square and the length held to a closer figure.

That matters in stacks, where individual length differences add up and tilt the assembly.

Cut tube spacers can be perfectly adequate and are cheaper, particularly where the exact gap is not critical.

The tolerance belongs to the part number, not to the section shape, so it has to be read.

For precision assemblies, buying all spacers from one batch reduces variation further than the stated tolerance suggests.

On polymer spacers easily, and on soft metals with the wrong tool.

Nylon and similar polymers round off quickly if a spanner is used with real force, after which the spacer cannot be held at all.

The spacer is not meant to take tightening torque - it is held to stop it turning, not to resist the fastener.

Using a correctly sized spanner rather than adjustable jaws prevents most of the damage.

In brass and aluminium the flats deform rather than round off, which is usually cosmetic.

If real torque has to be reacted through the spacer, the design should be using a standoff instead.

Panel and enclosure mounting, board stacks assembled by hand, and anywhere the spacer must be held during assembly.

Enclosure work often involves reaching behind a panel with one hand, where a spinning spacer is genuinely difficult.

Multi-layer board assemblies benefit from the better length tolerance as well as the handling.

Assemblies that will be dismantled repeatedly are easier when the spacer can be held rather than dropped.

Visible assemblies sometimes use hex sections for appearance, matching the fastener heads.

Where space is tight or the spacer is invisible and captive, round is cheaper and equally effective.

The same range as round spacers - brass, aluminium, steel, stainless and various polymers - chosen for the same reasons.

Brass is the traditional choice for machined hex spacers, being easy to machine and corrosion resistant.

Aluminium is lighter and cheaper by volume; stainless is used where the environment is demanding.

Nylon and glass-filled polymers give electrical isolation and low weight, at the cost of softer flats and creep under sustained load.

Where the spacer must break a galvanic path between two metal parts, a polymer spacer is doing real work.

Plating on metal spacers should be compatible with the fastener and the parts, since it contacts both.