The company that sells both handle shapes says the choice between them is strictly a personal preference. That line comes from Butterfly's own equipment FAQ, and it takes away the one thing most sample review forms exist to produce: a winner. If shape is a preference, no approval meeting can settle it by argument, and a buyer who signs off on ST because it looks like the neutral default has not made a technical decision. They have made an unrecorded one.
What a sample pack can actually do is narrower. Butterfly's guidance gives you two labels, FL and ST, and a plain description of the geometry behind each. Those labels belong on the sample form beside a tester's name, not in a ranking column. What usually fails in a custom paddle approval is not the shape. It is the missing record of who held which shape, and why they preferred it.
Why the Handle Spec Cannot Pick a Winner
The published position is short: which handle style is right is strictly a personal preference, and many blades come in different handle styles. Read that as a procurement document rather than a consumer tip and it changes what the sample round is for. A preference statement carries no ordering. It does not say FL fits more hands, or that ST suits a particular playing style better, or that one shape is less tiring over a long session. It says the answer sits with the person holding the paddle.
That has a direct consequence for how you build the review. A scoreboard asks testers to rank two shapes and then averages the ranks, which produces a number the manufacturer's own wording does not support. A recording instrument asks each tester which shape they held, what they noticed at the base, the waist and the neck, and what reason they gave for preferring one. The second format produces something you can defend in a supplier conversation. The first produces a preference dressed up as a specification.
Keep the scope tight while you use it. This is Butterfly's guidance for its own custom racket configuration, written for people choosing among blades that company sells in several handle styles. It is not an industry survey, not a test report, and not a statement about what any contract factory offers or how its handles are shaped. Use it for the vocabulary and the preference framing. Do not use it as evidence that your supplier's FL is the same FL. The label set travels; the fit verdict does not.
FL and ST Shapes in the Manufacturer's Words
Both definitions are worth keeping verbatim on the form. A (FL) handle is wide at the base and is concave in the center. The (ST) handle has no deviation in it and is uniform from neck to base. Those are geometry statements, and they translate into things you can inspect with your hands and eyes rather than argue about across a table.
Concave in the center means the handle's middle section curves inward relative to the ends, so the profile dips between the neck and the flared base. Wide at the base means the cross-section grows toward the butt end, finishing in a flare rather than a straight cut. On a physical sample, an FL profile should look pinched in the middle and heavier at the end. Run a thumb along the waist and the middle should read narrower than the butt.
ST is the opposite construction in wording: no deviation, uniform from neck to base. Its cross-section stays constant along the length, so the profile does not narrow at the waist and the base does not flare. Run your hand from neck to butt and there should be no change in girth to notice. That is what uniform is describing, and it is a difference you can see without instruments.
Two cautions on the labels themselves. FL and ST are the terms used in this manufacturer's own configurator guidance, and a supplier may use the same two letters for shapes that only roughly match, or different letters for the same geometry. And the descriptions are shape-only. They tell you what the outline does, not how the handle feels in a particular hand.
What the Shape Descriptions Still Leave Open
Read as data, these are qualitative shape labels from one manufacturer. That is a real thing to have and a limited thing to have. What they support is a shared vocabulary for describing both shapes on a sample form, plus a match check: does the object in the box look like the FL or the ST as described? If the supplier's spec sheet says FL and the sample shows a straight-sided handle with no flare at the base, the description and the object disagree, and that is worth a written question before anything else moves.
What they cannot support is a numeric acceptance line. There are no dimensions here, no width at base, no waist depth, no length, no weight, no tolerance, no surface finish, and no ranking between the two shapes. Nothing in those two sentences says how any individual hand will feel on either one. A form that converts these descriptions into pass or fail thresholds has invented the thresholds.
Any parameter layer that would let you write a real acceptance spec has to come from the supplier you are actually dealing with, and none of it is in hand yet. Until that material arrives, handle shape belongs in the approval record as a described and tested attribute, not as a tolerance. That is a weaker claim than a specification, and it is the honest one.
Three Change Points in One Sample Spec Sheet
Suppose your own handle line on the sample approval form goes through three drafts. It is a hypothetical document, not a record of anyone's product history, and it shows where the wording usually tightens.
Originally, the line read only FL handle, a name with nothing under it. After round one it was revised to carry the manufacturer's own wording for that shape instead of a bare label. That change alone made the form testable, because a reviewer could now compare the sentence to the object in front of them.
A further revision added the ST line in matching language, so both shapes in the sample pack were described in the same terms and neither read as the default. Then, before approval, the line was redesigned again to add two fields per shape: who tested it, and the reason that person gave. That third change follows straight from the preference framing. If the shape choice rests with the individual, the individual's identity and stated reason are the evidence.
No dates, version numbers or supplier-side history are claimed here. The three change points are visible in the document's own lineage: originally, revised after round one, redesigned before approval. If your form has not moved through something similar, it is probably still at draft one.
What Would Overturn the Preference-Only Conclusion
Here's the catch. Personal preference is the right basis for approval only while the shape stays a shape. Four findings would end it, and each one is a check you can run during the sample round rather than a claim about any factory.
The first is a written specification that assigns a fixed shape or a dimensional target instead of a shape name. If the supplier's document specifies a base width, a waist depth or a profile tolerance, the decision stops being a preference question and becomes a conformance question, and preference language would let a non-conforming sample through. The second is a conflicting published handle description from another manufacturer, one that frames selection as fitting a hand type or a playing style rather than as individual preference. That would not make the Butterfly wording wrong for Butterfly's own blades. It would mean the vocabulary you borrowed no longer covers the field you are buying in.
The third is a sample that does not match the geometry as described, an FL with no flare or an ST with a visible waist. Then the description is not labeling your object and the record behind it is worthless. The fourth sits in your own paperwork: an approval that ties acceptance to a chosen shape rather than to a recorded tester result. Ask yourself which of the four you have actually ruled out. If the answer is none, the preference framing is still a placeholder, not a conclusion.
Back to the Sample on Your Desk
The paddle in front of you does not need a verdict. It needs a description and a name next to it. Write the shape using only the two definitions that are actually supported, and then add the fields the manufacturer's own preference line makes necessary: which tester, which shape, what reason. A description plus a name is what makes the line auditable later, when nobody remembers who held the sample.
Then list what you do not know as questions rather than assumptions, and send them with the sample comments. Whether the quote covers the handle shape you tested or only a stock handle. The minimum order quantity and whether it changes with shape. Lead time, capacity, and how much control you have over the handle profile. Whether the supplier's spec sheet names shapes or fixed dimensions, because the answer decides which of the four checks above you are actually running.
Sign the sample off as tested by these people, described in these two manufacturer definitions, and reopen it if the supplier's written spec adds a fixed shape target or a second source contradicts the shape-only framing. The choice between FL and ST was never yours to make on a buyer's behalf. The record of who made it, and on what evidence, is yours to keep.