Show-through begins with light, not ink coming through the sheet.
When type or an image on the reverse can be seen through an unprinted area, the paper is not blocking all of the light reflected from the page beneath it. Paper opacity describes resistance to that transmission. A more opaque sheet generally keeps the reverse-side image quieter, which matters when a novel has dense text, a workbook uses rules and tables, or an illustrated book places a dark picture opposite a pale page.
That optical effect is different from ink penetrating deeply into the sheet. Printers distinguish show-through from strike-through because the remedies are not identical. Paper formation, fillers, coating, thickness and fibre composition can affect opacity, while the ink, print process and surface absorption influence how the printed film sits and dries. A page that looks busy from behind should therefore be investigated as a paper-and-print combination rather than dismissed as “paper too thin”.
GSM tells you mass; it does not tell you the whole reading experience.
Two papers with the same grammage can have different caliper, bulk, opacity, shade and surface. One may feel fuller in the hand because it has more volume for its weight. Another may be smoother and denser. That is why substituting a stock by matching only the number of grams per square metre can change the spine, page turn and amount of reverse-side image that remains visible.
Ask for the relevant paper data and a physical sample when the choice affects a substantial run. Compare weight, feel and binding together with the paper and binding options. For a book with a calculated spine, use the actual stock caliper or a printer-supplied template rather than assuming that every paper of a stated weight builds to the same thickness.
Page design can expose or soften the limits of a lightweight stock.
Large dark areas placed back-to-back are a demanding test. So are heavy rules, high-contrast photographs and solid chapter openers opposite generous white space. In a colour book, map image-heavy pages while the paper decision is still open. A page map, as described in the colour page planning notes, can reveal expensive or visually awkward combinations while the files are still flexible.
A more opaque paper is not automatically the only solution. The design may be able to avoid two dense images sharing a leaf, reduce an unnecessary solid background or use a different stock for a particular section. Those changes should serve the book rather than hide a weak specification. A text face, line weight or tonal image still needs to reproduce as intended on the chosen surface.
A representative two-sided proof is more revealing than a loose swatch.
A blank paper sample shows shade, texture and stiffness, but it cannot demonstrate the interaction between two printed pages. Select a spread containing small type, a dark image and a quiet area, and review both sides under ordinary reading light. If the proposed paper or production route is not available for the proof, record what the sample does and does not represent.
Handle a bound or folded dummy as well when thickness is part of the decision. It will not predict colour by itself, but it can expose a book that has become unexpectedly bulky, a page that feels too limp, or a gutter that needs adjustment. Use the soft proof and hard copy proof comparison to separate content approval from material and handling approval.