Cup
The faces curve across the width — like a shallow trough. Common on plain-sawn boards. Mild cup can joint out; deep cup costs a lot of thickness.
What Are Bow, Cup, Crook, and Twist in Hardwood?
Surfacing two faces does not make a board flat. Cup, bow, crook, and twist (also called wind) are different ways a board can warp — and a thickness planer alone cannot fix them. At Fells Hardwood Supply in Hutto, we walk shoppers through these shapes before they mill, so they buy enough wood and joint the right face first.
The four shapes to look for
Hold the board at eye level, sight down the faces and edges, and set it on a flat bench. You are looking for whether the faces and edges are true — not whether the faces look smooth after surfacing.
Bow
The face curves along the length — like a banana. Sight down the face from the end. Mild bow joints out; severe bow may waste more wood than the board is worth.
Crook
The edge curves along the length while the faces may still look fairly flat. You see it when you sight down an edge or try to rip a straight fence line. Joint or rip one edge straight before you trust widths.
Twist / wind
The four corners are not in one plane — the board rocks on the bench. Wind is only one kind of warp, not a synonym for every problem. Twist is often the most expensive to flatten.
Photo slot: Real boards from the store labeled cup, bow, crook, and twist can replace these diagrams later. Until then, the SVGs teach the shapes.
How do you flatten a warped board on the jointer?
Start by making one face truly flat. That face becomes your reference. Then thickness-plane the second face parallel to it. Next, joint one edge straight against that flat face. Rip the second edge parallel. Do not assume an S2S board is already flat — a thickness planer follows the board’s existing shape; it does not create a true face the way a jointer does.
Which face should I joint first?
Prefer the hollow (concave) face against the jointer bed when the machine and board allow it. High spots meet the knives first, the board stops rocking, and you remove less wood than chasing a crowned face that keeps tipping. For twist, take light passes and stop when the face sits flat — then move to the planer. If the board is too bad to save, put it back and pick another; that is cheaper than planing it into a thin scrap.
Does S2S mean the board is flat?
No. S2S means both faces were planed to a thickness. It does not mean the board is free of cup, bow, crook, or twist. Moisture change after milling can also bring warp back. For furniture glue-ups, plan to true faces again. Tags that say S3S or S2S SL1E (straight-line one edge) only add one straight edge — they still do not guarantee a flat face.
How much extra wood should I allow for warp?
Buy starting thickness for the finished size you need, plus room for jointing, planing, and a final skim. Mild cup or bow may cost 1/16"–1/8"; bad twist can eat far more, or make the board unusable. Add about 15–30% to your board-foot estimate for defects, milling loss, and miscuts — more when the stock is wild. We sell lumber in standard board foot pricing. Widths and lengths vary per board, and if you need specific sizes we can mill boards for you at a fair rate.
Related: Thickness and milling terms · Rough, S2S, S3S, S4S · Kiln dried, defects, and drying · All FAQs
Fells Hardwood Supply — Hutto, TX — serving Austin, Round Rock, Cedar Park, Georgetown, and the greater Austin metro.