This article walks through the four fixed references every modern compound bow shares: the AMO draw length standard, brace height, the Berger hole, and standard center shot. All four are standardized. All four have real mechanical meaning. And at least one of them — the AMO draw length — is misunderstood often enough that a lot of tuning problems get chased for weeks that a two-minute geometry check would have solved.
AMO draw length, and the 1.75" oddness
If you put a bow labeled 28" draw length on a draw board and measure it correctly, the bow will read 26.25". The bow is not wrong. The label is not wrong. What is happening is the industry's oldest and most confusing measurement convention.
Draw length on a compound bow is reported under the AMO standard (Archery Manufacturers Organization, later ATA — Archery Trade Association). The rule is:
AMO draw length = (distance from grip pivot to the deepest point of the nock groove, at full draw) + 1.75 inches.
The measurement you take on the draw board — pivot point of the grip, to the inside of the nock groove on the string, at full draw — is the actual physical draw stroke. Add 1.75" to that number and you get the AMO draw length, which is the number stamped on the module, printed in the spec sheet, and used to select every arrow, sight, and stabilizer accessory built for the bow.
Concrete: mods set to 28" AMO draw length will measure 26.25" pivot-to-nock on the draw board. That is not an error. That is the standard working as designed.
Where the 1.75" came from
The 1.75" is not arbitrary and it is not a fudge factor. It comes from AMO-era recurve bows, where arrow length was measured to the back of the bow — the outside face of the riser facing the target. On those bows, the distance from the grip pivot to the back of the riser was standardized as 1.75", which meant an archer with a 28" AMO draw length could shoot a 28" arrow measured to the back of the bow and know the arrow tip cleared the riser at full draw. The standard let arrow manufacturers, bow manufacturers, and archers use one number instead of three.
Modern compound risers do not have the same geometry — the pivot-to-back distance varies bow to bow — but the AMO number stuck because it is what every module, spine chart, arrow-selection chart, and spec sheet is built around. Change the standard now and every reference chart in archery breaks. So the 1.75" persists, decades after the bow it was designed for.
The practical implication for a technician: never trust a labeled draw length without verifying it on the draw board. Modules can be mis-set from the factory. Rotating mods drift when they loosen. Measure the pivot-to-nock at full draw, add 1.75", and compare that to the labeled number.
Brace height
Brace height is the distance from the deepest part of the grip — the pivot point, where the archer's hand loads the riser — to the string at rest. Unlike draw length, there is no offset, no standard adder, no historical quirk. What you measure is what you report.
The mechanical meaning of brace height is the length of the power stroke — the distance the string travels from rest to full draw, which sets how long the string is in contact with the arrow during the shot. Every millimeter of power stroke is stored energy transferred to the arrow. Longer brace height means shorter power stroke: less energy, more forgiveness, more time for the arrow to leave the string cleanly before shooter error is expressed. Shorter brace height means longer power stroke: more energy, more speed, and less tolerance for the archer's imperfections.
The trade between forgiveness and speed lives inside brace height. That is why modern hunting compounds typically sit at 6" to 7" brace while target compounds run 7" to 8". It is not fashion. It is the shooter–speed tradeoff, expressed in a single dimension.
Measurement is straightforward: bow square across the deepest part of the grip, extend to the string at rest, read. The only place people get this wrong is when they measure from the front face of the grip instead of the pivot — which will give a number that is a few tenths short. On grips with an aggressive throat, pay attention to where the archer's hand actually loads. The pivot is where the palm contact bottoms out, not where the grip material starts.
The Berger hole
The Berger hole is the small threaded hole in the riser, just above the shelf, that holds the arrow rest. Every modern compound bow has one, in essentially the same location relative to the shelf. It is the single most standardized reference point on the entire bow.
The hole is named after Vic Berger, a tournament archer in the late 1960s and early 1970s who designed the Berger button — a spring-loaded plunger used to tune arrow flight on recurves. The button needed a hole to mount in. The hole ended up in a specific location for good mechanical reasons: it sat at the height of the arrow shelf, on the pressure-point axis of the arrow, aligned vertically with the grip pivot below it. Over the next decade the button faded from most compound setups, but the hole stayed. Modern arrow rests — drop-away, blade, containment, all of it — mount through the same threaded hole Vic Berger specified for a completely different device.
The thread standard is 5/16"–24 UNC. That has not moved either. A rest from any modern manufacturer will thread into a Berger hole on any modern riser without an adapter. That kind of cross-brand mechanical compatibility is rare in archery, and it exists because the hole never got un-standardized.
The Berger hole matters for the entire mechanical system for three reasons:
1. It defines the arrow-flight axis. The vertical position of the Berger hole is where the arrow shaft sits when a rest is mounted. That axis is the reference plane for cam-lean analysis, nock height setup, and any diagnostic that needs to know where the arrow actually is in space.
2. It is the reference for cam alignment. When a technician checks cam lean, the reference is the plane defined by the string and the Berger hole axis. Cams that lean in that plane are correctable through yoke tuning; cams that lean out of that plane point at a different problem.
3. It anchors the horizontal reference for center shot. Center shot is measured from the riser to the arrow center, and the arrow's height is fixed by the Berger hole. Move the hole, and every downstream measurement moves with it. It does not move, so everything else has a stable place to be measured against.
An easy way to think about it: the Berger hole is the origin of the coordinate system every modern bow is designed around. Everything else — grip shape, cam profile, riser cutout — varies. The hole does not.
Standard center shot, 13/16"
Center shot is the distance from the inside face of the riser to the center of the arrow shaft, measured with the arrow resting on the mounted rest. On a modern compound bow, the industry-standard starting point is 13/16" (0.8125").
Measuring center shot correctly is exact work. Measure the outside diameter of the arrow shaft with calipers. Divide by two. Add 13/16" (0.8125"). That is the total distance from the riser to the outside edge of the arrow on the riser side. A cheap way to verify: use a straightedge from the string down through the arrow rest and confirm the arrow crosses the reference plane where you expect it to. The number is not negotiable at the starting stage; the adjustments come later, through tuning.
Why this matters when things go wrong
Every serious tuning problem has a geometry check that could have caught it before the twist-cables-and-hope stage began.
Left/right groups drifting? Confirm center shot is 13/16" before adjusting anything. If it is 3/4" you have a fletching clearance issue that no amount of tuning will silence. If it is 7/8" the string path is probably not what you thought it was.
Vertical group drift under different draw pressures? That is often cam timing, which is measured at the draw board — but the draw board itself references the string against the Berger hole axis. Get the axis wrong and the timing measurement is wrong.
Draw length "feels short" and the archer is over-extending? Verify AMO draw length on the draw board first. Pivot-to-nock at full draw, plus 1.75". If the module is labeled 28" and the board reads anything other than 26.25", the strings might not measure what you think they do.
Arrow spine feels off across the whole setup? Confirm draw length is what the archer thinks it is, in AMO terms. An archer who has been shooting a 28" mod that is actually cranked to 28.5" is at a different power stroke than the spine chart assumes, and every arrow selection downstream is off.
The fixed references are not just numbers. They are the shared vocabulary the bow is built in. Speak that vocabulary and the bow will tell you exactly what is wrong when it is not working. Ignore it, and every problem looks like every other problem.
Published 2026-07-04 · Axial Bowstrings