The tilt mechanism is the steel housing bolted between the seat pan and the gas cylinder, and the four names on a listing describe where that housing puts the pivot point. Move the pivot and you change what happens to your feet when you lean back. That is the whole subject, and almost no listing says it.
This page names the four common mechanisms, says what pivots where in each, and explains the one number that appears alongside them, the synchro ratio. It does not tell you which to buy, because the honest answer depends on your desk and your legs, and because the word on the listing is not always attached to the same hardware.
The pivot point is the entire difference
Every reclining office chair is a lever. Something has to stay still while something else moves. Where the manufacturer puts the fixed point decides three things at once: whether the seat tips, whether your feet stay on the floor, and whether the front edge of the seat rises into the back of your thighs.
Read the four mechanism names as four answers to that one question.
Center-tilt, also sold as single-point tilt
The pivot sits roughly under the middle of the seat. Lean back and the whole assembly, seat and backrest together as one rigid unit, rotates around that central point. The backrest goes back, the rear edge of the seat goes down, and the front edge of the seat comes up.
That last part is the consequence people notice. On a center-tilt chair, reclining lifts your heels. If the chair is set to the height where your feet are flat, they will not be flat once you lean.
This is the simplest and cheapest housing to build, which is why it appears on the majority of chairs at the low end of the market and on almost every conference and guest chair that tilts at all.
Knee-tilt
Same rigid seat-and-back unit, but the pivot moves forward, to roughly where your knees are. Lean back and the assembly still rotates as one piece, but because the fixed point is now at the front, the front edge of the seat barely moves.
Your feet stay where they were. The rear edge of the seat drops further than it would on a center-tilt to produce the same backrest angle, so a knee-tilt chair usually feels like it opens up more for the same amount of lean.
Knee-tilt is common on executive-category chairs and on anything sold for long recline sessions. It is a more expensive housing than a center-tilt, because the load path from the pivot to the cylinder is longer and the casting has to carry it.
Synchro-tilt
Here the seat and the backrest stop being one piece. The backrest reclines through a larger angle than the seat does, and the two move together in a fixed proportion. The mechanism performs two rotations at once, which is where the name comes from.
The point of it is that the backrest can travel a long way back while the seat stays close to level, so the angle at your hip opens without your thighs being tilted up toward your chest.
Synchro is the mechanism most often named in ergonomic marketing, and it is also the one where the listing tells you the least about what you are actually getting, because of the ratio.
Multi-function, also sold as multi-tilt
Multi-function is not a fifth geometry. It is a housing that adds independent controls on top of one of the above: usually a separate backrest angle lock and a separate seat angle adjustment, so the seat tilt and the back angle can be set to values that are not linked to each other.
This is the word that varies most between makers. Some use multi-function to mean a synchro mechanism plus a seat angle adjuster. Some use it to mean a knee-tilt plus a back angle lock. Some use it to mean any mechanism with three or more levers. A reader comparing two chairs both labeled multi-function is not necessarily comparing the same hardware, and neither listing will say which one it means. This is the same failure the armrest scale has, where the fourth axis is defined differently by different manufacturers and the number on the listing turns out not to be a specification at all.
The synchro ratio, and what it does not tell you
Synchro chairs are advertised with a ratio. Two to one and three to one are the two you will see most.
The ratio is the proportion between backrest travel and seat travel. On a 2:1 mechanism, for every two degrees the backrest goes back, the seat tilts back one degree. On 3:1, the seat moves one degree for every three of backrest.
That is genuinely useful, and here is the limit on it. The ratio is a proportion, not a range. It tells you how the two surfaces move relative to each other. It does not tell you how far either of them goes.
A 3:1 mechanism with a small total backrest travel moves the seat less in absolute terms than a 2:1 mechanism with a large total travel. Two chairs with the same advertised ratio can end up in visibly different positions at full recline. To compare them properly you need the ratio and the total backrest travel, and the total travel is the number that goes missing from listings far more often than the ratio does.
There is a second trap underneath that one. Backrest travel is published against two different conventions that sit ninety degrees apart, so the same recline can be written as fifteen degrees or one hundred and five degrees depending on which baseline the maker used. Even when a total is published, you have to work out which convention produced it before you can set it beside anyone else's number.
What the mechanism name does not include
Three things live in or on the tilt housing that the mechanism name says nothing about.
Tension. The knob under the seat that decides how much of your weight it takes to start the recline. That is a spring preload adjustment and it is separate from the geometry. So is the lever that stops the recline at a chosen point. Tension and lock are two different controls doing two different jobs, and a chair can have either, both or neither regardless of which of the four mechanisms it uses.
The tilt limiter. A third control, sold separately from tension and lock by the manufacturers that offer it, which restricts the recline to a chosen fraction of its full travel rather than pinning it at one fixed angle. Listings routinely fold limiters in with locks. They are not the same part and they do not produce the same result.
The seat slider. Seat depth adjustment is a rail assembly that sits on top of the mechanism rather than inside it, and it is specified, and often priced, as a separate option.
How to read the listing
In order, and stopping when the listing runs out of information:
- Find the mechanism word. Center, single-point, knee, synchro, multi-function. If there is no word at all, assume center-tilt, because that is what costs least to fit.
- If it says synchro, look for the ratio, then look for the total backrest travel. The second number is the one that lets you compare two chairs.
- If it says multi-function, read the list of controls rather than the label. Count the levers and find out what each one does.
- Check separately for tension, lock and limiter. None of them is implied by the mechanism name.
- Accept what is missing. Most listings do not publish total travel, spring range, or the occupant weight range the tension was designed around. That absence is the normal condition of this market, not evidence of a bad chair.
If you want the wider version of this exercise, the full walk through an office chair listing line by line covers the rest of the spec block the same way.
FAQ
What is the difference between synchro-tilt and knee-tilt?
They differ in whether the seat and backrest are one piece. On a knee-tilt the two are a rigid unit that rotates around a pivot placed roughly at your knees, so the front edge of the seat barely moves while the rear edge drops. On a synchro-tilt they are separate: the backrest reclines through a larger angle than the seat, and the two move together in a fixed proportion, which is what the ratio describes.
What does a 2:1 or 3:1 synchro ratio mean?
It is the proportion between backrest travel and seat travel. On a 2:1 mechanism the seat tilts back one degree for every two degrees of backrest, and on 3:1 one degree for every three. It is a proportion and not a range, so it does not tell you how far either surface actually goes. A 3:1 mechanism with small total backrest travel moves the seat less in absolute terms than a 2:1 with large total travel, which is why you need the total as well as the ratio.
What does multi-function mean on a chair listing?
Not a fifth geometry, but a housing with independent controls added on top of one of the other three, usually a separate backrest angle lock and a separate seat angle adjustment. The word varies more than any other in this category. Some makers use it for a synchro plus a seat angle adjuster, some for a knee-tilt plus a back angle lock, some for any mechanism with three or more levers. Read the list of controls rather than the label.
My listing does not name a tilt mechanism at all. What does that suggest?
Assume center-tilt, also sold as single-point tilt, because it is the simplest and cheapest housing to build and it is what appears on most chairs at the low end of the market and on almost every conference and guest chair that tilts. It is also the geometry where reclining lifts your heels, since the pivot sits under the middle of the seat and the front edge rises as the back goes down.
Is tilt tension included in the mechanism type?
No. Tension is a spring preload adjustment, set by a knob under the seat, that decides how much of your weight it takes to start the recline, and it is separate from the geometry. So is the lock, which stops the recline at a chosen point, and so is the tilt limiter, which restricts recline to a fraction of its travel and is a distinct part that listings routinely fold in with locks. A chair can have any, all or none of the three whichever mechanism it uses.
The one honest summary
Center-tilt lifts your feet. Knee-tilt does not. Synchro keeps the seat closer to level while the back goes further, in a proportion the ratio describes and a distance it does not. Multi-function means extra levers, and which extra levers depends entirely on who wrote the listing.
Everything else in this category is a word a marketing department chose, and the word is not the hardware.