58The requirement for the Commission to specify the subject-matter and purpose of the inspection therefore constitutes a basic guarantee of the rights of defence of the undertakings concerned and consequently the scope of the duty to give reasons for decisions ordering inspections cannot be limited by reference to considerations relating to the effectiveness of the investigation. In that connection, whilst it is true that the Commission is not required to communicate to the addressee of such a decision all the information at its disposal concerning the presumed infringements, or to delimit precisely the relevant market, or to set out the exact legal nature of the presumed infringements, or to indicate the period during which those infringements were committed, it must none the less state as precisely as possible the presumed facts which it intends to investigate, namely what it is looking for and the matters to which the investigation must relate (see, in relation to Regulation No 17, Case 85/87Dow Benelux v Commission [1989] ECR 3137, paragraph 10; Hoechst v Commission, cited in paragraph 57 above, paragraph 41, and Roquette Frères, cited in paragraph 29 above, paragraph 48).