Reading the prep as one dimension from the edge
Everything on this page reduces to a single number: how far in from the door edge the prep reaches. For a bored lock it is the backset plus half the cross bore — 2-3/8 plus 1-1/16 is 3-7/16. For a mortise lock it is the case depth off the template, which is always deeper than the backset. For an exit device it is whatever minimum stile the template names.
Against that sits the solid material, which is the stile width less however far the glass or panel groove cuts back into it. A 4-1/2 stile with a half inch groove has 4 inches of solid. The 3-7/16 prep leaves 9/16 behind it, which is comfortable. Order the same lock in a 2-3/4 backset and it leaves 3/16, which is not.
The rose overhangs before the bore breaks through
Trim is usually the first thing to fail on a narrow stile, and it fails silently — the hole is fine, the lock works, and the escutcheon sits over the glass bead looking wrong forever. A 2-3/4 rose on a 2-3/8 backset reaches 3-3/4 from the door edge, so anything narrower than that stile-wise has trim hanging past the inner edge. Long escutcheon trim reaches further still, which is why it shows up on wide stile doors and rarely on narrow ones.
Backset is the cheap variable
Of everything in the form, backset is the one that costs nothing to change at order time and is expensive to change afterwards. Most bored locks ship in one of two backsets and many have an adjustable latch that does both. Six eighths of an inch either way is often the whole difference between a prep with room and a prep that clips the groove, and the page prints both so the choice is made before the hole saw comes out.
Where the page stops
It checks dimensions and nothing else. It has no view on whether the lock suits the opening, whether the device is the right function, or what the fire rating on the leaf permits — and that last one matters more than the rest put together, because boring or mortising a rated leaf outside what its listing already allows voids the listing and turns a repair into a replacement. If the opening is rated, the question goes to the manufacturer of the assembly first. Related: the door hardware schedule calculator for the counts across a set of openings, and the strike lip length and frame prep calculator for the other half of the same prep.
Questions people ask
What stile width does a standard bored lock need?
Work it from the prep rather than from a rule of thumb. A 2-1/8 cross bore at a 2-3/8 backset reaches 3-7/16 in from the door edge, so the solid part of the stile has to be at least that before the groove starts. At a 2-3/4 backset it reaches 3-13/16. Add whatever the groove cuts back and you have the stile width, and the trim usually wants a bit more than the hole does.
Why does the rose overhang when the hole fits?
Because the trim is bigger than the bore and it is centred on the same point. A 2-3/4 rose at a 2-3/8 backset has its outer edge 3-3/4 in from the door edge, while the hole only reaches 3-7/16. On a 3-1/2 stile the hole is fine and the rose sits a quarter inch over the glass bead. It is a cosmetic failure rather than a functional one, which is exactly why it survives the installation and annoys people afterwards.
Is 2-3/8 or 2-3/4 backset better?
Neither, and the choice is usually made by the stile rather than by preference. The shallower backset leaves more material before the groove, which matters on narrow stiles; the deeper one moves the lever further from the jamb, which matters where knuckles are hitting the frame or where an existing prep has to be matched. Many bored locks ship adjustable between the two, so the cheap move is deciding before ordering.
Can I bore a fire-rated door?
That question does not get answered here. Cutting, boring or mortising a rated leaf outside what its listing already permits voids the listing, and a voided assembly is replaced rather than repaired. What the listing allows for that specific door comes from the manufacturer of the assembly, and this page gives no method, no dimension and no way round it.
Does a mortise lock need more stile than a bored lock?
Usually yes, because the case is a pocket rather than a hole and the pocket runs the height of the case. A 3-3/4 case depth on a 4-1/2 stile with a half inch groove leaves a quarter inch of solid behind the case, and that quarter inch is holding the whole inner half of the stile together. A bored lock that reaches the same depth still leaves material either side of the hole.