57It states, further, that the market in 4 mm float glass must be considered to be the relevant product market as that product cannot be substituted by float glass of other thicknesses: the cross-elasticity of demand between 4 mm float glass and float glass of other thicknesses is zero; increases in the price of 4 mm float glass are unlikely to have any effect on demand for other float glass products. In that regard, although there is significant fluctuation in the price charged for 4 mm float glass in Ireland, demand for other float glass products has remained constant. According to both the case-law of the Court of Justice and Court of First Instance and the decisions of the Commission (Commission Decision 88/138/EEC of 22 December 1987 relating to a proceeding under Article 86 of the EEC Treaty (IV/30.787 and 31.488— Eurofix — Bauco/Hilti) (OJ 1988 L 65, p. 19); Commission Decision 92/163/EEC of 24 July 1991 relating to a proceeding pursuant to Article 86 of the EEC Treaty (IV/31.043 — Tetra Pak II) (OJ 1992 L 72, p. 1); judgment in Case C-53/92 PHilti v Commission [1994] ECR I-667; judgment in Case T-30/89Hilti v Commission [1991] ECR II-1439; judgment in Case T-83/91Tetra Pak v Commission [1994] ECR II-755), there is a relevant product market when cross-elasticity with other products, which may be considered interchangeable, is low: it follows that a product market is a fortiori distinct from another where the cross-elasticity between them is zero.