A linear foot (or lineal foot) is just length — 12 inches along the piece, cross-section ignored. Total up pieces, price them at $/LF, and flip the total into board feet when a lumber quote needs comparing.
Anything with a fixed profile sells by length: baseboard and casing, decking, fence boards, gutters, countertops, even moving-truck freight. The unit works because the cross-section is already decided — the only question is how many feet of it you need. For a room's trim, measure the perimeter and subtract door openings; for a deck, multiply board count by length (and let the BF ↔ LF converter handle the lumber-order math).
Trim quoted at $/LF and rough stock quoted at $/BF can describe the same wood. To compare, convert: BF = LF × width″ × thickness″ ÷ 12. Forty-eight linear feet of 1×6 is 24 board feet; if the trim costs $2.40/LF, that's $4.80 per board foot — now it lines up against the hardwood price table. The reverse trip (how long a board a given volume makes) is the same formula backwards.
Yes — "linear" only clarifies you're measuring length along the piece, not area or volume.
Perimeter 52 ft minus openings — with one 3′ door, order ~49 LF plus 10% waste ≈ 54 LF.
The estimator's loop: walk the room once per profile. Baseboard = perimeter minus openings; casing = 17 LF per standard door (both sides) and ~14 LF per window; crown = the full perimeter, openings included. A 12×14 bedroom with one door and two windows totals roughly 49 LF base + 17 LF casing + 52 LF crown — order in the standard 8/12/16-foot sticks that minimize joints, and add 10% for miters. Price each profile at its $/LF above, or convert to board feet (BF = LF × W″ × T″ ÷ 12, per the nominal convention in NIST PS 20) when you're deciding whether to buy trim or mill it from rough stock — the spread on paint-grade poplar is often 2–3× in favor of milling it yourself.