What is a Sidebar?

A sidebar is a locking mechanism used in some higher-security locks -- Medeco, ASSA, Schlage Primus, some of the higher-security automotive cylinders, and so on. They aren't limited to pin tumblers but they are limited to locks with a shear plane, so they wouldn't be used in lever locks.
The Medeco's a good one to look at to understand the idea.
Think about how a normal pin cylinder works: All the pins have to be lifted to the right heights to create a shear plane that permits the plug to rotate.
The sidebar is another piece of metal which extends from the plug into the shell, blocking rotation just as the pins do. The difference is that it is a single piece, typically running the full length of the plug, and has "fingers" (protrusions) on it (think of the shape of the letter E) which extend to and are blocked by the pins. To release the sidebar, the Medeco pins must be rotated to the right orientation, permitting a slot or hole (depending on the version) to line up with and accept the sidebar's fingers. Unless they _all_ line up, the sidebar can't withdraw far enough into the plug to create a shear line and permit it to turn.
In general, sidebar locks tend to be more pick-resistant than earlier designs.

Here's a somewhat "Out of the box" explanation.
A sidebar is a "bar at the side". It's yet another gadget in there, each of which makes the lock more resistant to picking. You can work out the rest by Googling for a picture.
But the interesting part is more subtle. A pin cylinder can be picked because you can (by and large) attack each pin in turn. Picking one pin isn't hard, and if you can repeat this in the right order, then you have the whole cylinder.
A sidebar is a whole different problem. Like the floating lever in a combination lock (why you can't "hear tumblers falling in one by one", as the films have it), a sidebar doesn't even think about moving into engagement / release under the cylinder is turned. So you don't have the chance to attack each element of it individually - the pins are put into place for it, then the sidebar is tested against them together. It's this inability to attack it piece-by-piece that makes picking a sidebar lock such a considerable jump in difficulty.

Sidebars are used in some lever tumbler locks most notably Chubb 'Castle' and also 'Butters System' masterkeyed locks (the latter may now be of historic interest only). Normal lever tumbler locks cannot be masterkeyed in a similar manner to pin tumbler locks because of the nature of their design (an inferior method of masterkeying uses a 'master' lever which lifts other levers along with it - there is a high chance that a masterkey for one suite may fit other nearby suites). In these sidebar lock types, the sidebar finger enters a slot cut into the edge of the lever. There can be a second slot for masterkeying and one or more shallow slots to deter picking. I think I am correct in saying that in Butters System lock, the bolt operates the sidebar as the key moves the bolt, whereas in the Castle lock, the sidebar is moved by the curtain (which rotates with the key) . Someone may be able to describe it better.
Some brass 'pancake' padlocks (made under numerous brands) also used a sidebar rather than levers which merely 'hooked' the shackle shut.

Hi Peter,
Some terminology differences between the UK and US. In the US, that part you are calling a sidebar that enters the gate in a lever is called a fence.
I think the important feature of a sidebar in the US is that it is a secondary locking mechanism that may or may not interact with the primary tumblers. For example, the sidebar in a Medeco or the one in a Lori Bell type cylinder interacts with the primary tumblers. In a GM door or ignition lock made from 1935 up until sometime in the 80's or 90's for many cars the sidebar is the only locking mechanism. In an ASSA Twin of the various varieties the sidebar interacts with a set of secondary tumblers. In all of those cases the sidebar is as defined in the LIST Council Locksmith Dictionary at the link. BBE.
> > > >Hi All, > > > >What are "sidebars" and how do they work? Are they found only in pin > >cylinders or are they also used in lever tumbler locks? Sorry if this > >sounds like a silly question; obviously, I'm not a locksmith. Thanks > >again, > > > Sidebars are used in some lever tumbler locks most notably Chubb > 'Castle' and also 'Butters System' masterkeyed locks (the latter may > now be of historic interest only). Normal lever tumbler locks cannot > be masterkeyed in a similar manner to pin tumbler locks because of the > nature of their design (an inferior method of masterkeying uses a > 'master' lever which lifts other levers along with it - there is a > high chance that a masterkey for one suite may fit other nearby > suites). In these sidebar lock types, the sidebar finger enters a > slot cut into the edge of the lever. There can be a second slot for > masterkeying and one or more shallow slots to deter picking. I think > I am correct in saying that in Butters System lock, the bolt operates > the sidebar as the key moves the bolt, whereas in the Castle lock, the > sidebar is moved by the curtain (which rotates with the key) . > Someone may be able to describe it better. > > Some brass 'pancake' padlocks (made under numerous brands) also used a > sidebar rather than levers which merely 'hooked' the shackle shut.

To add to Billy's comment, the GM sidebar locks are wafer locks.
Leon Rowell
Billy B. Edwards Jr. wrote:

so are some of the Ford's.. LOVE Focus.. can you say JOB SECURITY??? drilled 2 out Friday.. bad sidebar. --Shiva--

And the sidebar operates the opposite way to the pin tumbler types.

Thanks for the replies! -C.W.