Sponge: maximum total thickness and what it implies
The ITTF rules fix a figure the player finds quoted everywhere, almost always without the clarification that gives it meaning: 4.0 mm. What those four millimetres limit is not the thickness of the sponge, but the total thickness of the covering glued to the blade. That covering includes three layers: the glue, the sponge and the topsheet. The sum of the three is what may not exceed the ceiling, measured on the finished bat.
The distinction matters because catalogues label rubbers by the thickness of the sponge alone — “Tenergy 05 in 2.1 mm”, “Rasanter R47 in 2.0 mm” — not by the total thickness. The topsheet already occupies between 1.6 and 1.9 mm depending on the model, and the glue adds its fraction of a tenth. That is why rubbers labelled “MAX” mount sponges of between 2.1 and 2.3 mm: beyond that figure, added to the topsheet and the glue, they would exceed the legal limit. Two rubbers both labelled “2.1 mm” can have slightly different total thicknesses if their topsheets differ in thickness. For the rules, only the sum counts.
The figure was fixed in the late nineteen-fifties, when sponge was starting to become widespread and some manufacturers were offering thicknesses that threatened to turn the rubber into the player's arm. The ITTF closed that path with a single ceiling and has maintained it for more than six decades. The entire market has structured itself around that figure.
Within the ceiling there is room for a wide range of decisions, all of them legal. At the upper end, thick sponges — around 2.1 mm — dominate the modern attacking game: maximum compression, greater energy storage. At the lower end, thin sponges — 1.5 mm, 1.3 mm — cut back the compression and are reserved for defensive profiles, short pimples or close blocking. And at the absolute limit is the rubber without sponge, OX, with the topsheet glued directly to the blade: a configuration almost exclusive to long pimples, for reasons developed in 6.3. The playing implications of each sponge thickness are covered in 5.7.
A point that seems minor but has practical consequences: the glue contributes to the total thickness. A generous layer can bring the assembly close to the ceiling. Something analogous happens with boosting — covered in chapter 14 and in 3.5 — which swells the sponge beyond its nominal value. In competition, umpires have instruments for measuring the bat — as is taken up again in 3.6 — and a rubber fattened by the treatment can mean disqualification without any other rule having been broken.
The 4.0 mm are, then, a ceiling on the complete covering, not on the sponge; the catalogue figure refers to only one of the three layers the ITTF measures; and within that ceiling, the choice of thickness is a playing-profile decision the rules permit but do not guide.