Opening and closing device, of the zip fastener type, including: - a zip having two connection bands each provided with a plurality of teeth, each tooth having a first fastening member provided on a first face of the tooth and a second fastening member provided on a second face of the tooth opposite to the first face, and each tooth of one of the connection bands being configured to cooperate with at least one tooth of the other one of the connection bands, via the first and second fastening members, so as to form links; and - a slider provided with a first opening located at a first end, with two second openings superimposed and located at a second end opposite to the first end, with a top ramp extending from the first opening as far as one of the second openings, and with a bottom ramp extending from the first opening as far as the other one of the second openings; with the opening and closing device being configured so that, under an action of moving the slider in a predetermined so-called opening direction, the teeth that cooperate enter the slider through its first opening, separate and travel respectively along its top and bottom ramps until they emerge from the slider respectively and individually through its two superimposed second openings; with the opening and closing device being configured so that, under an action of a movement of the cursor in a so-called closing direction opposite to the predetermined opening direction, the teeth of the connection bands individually and separately enter through the two respective superimposed second openings of the slider, and respectively travel along top and bottom ramps until they start to cooperate at the first opening of the slider; and with the opening closing device being characterised in that each tooth is provided with at least one first rounded portion on its first face and distinct from the first fastening member and/or with at least one second rounded portion on its second face and distinct from the second fastening member, characterised in that the first fastening member is formed by a protuberance, hemispherical in shape, provided projecting on the first face of the tooth and the second fastening member is formed by a hemispherically shaped housing, provided as an indentation on the second face of the tooth. - a zip having two connection bands each provided with a plurality of teeth, each tooth having a first fastening member provided on a first face of the tooth and a second fastening member provided on a second face of the tooth opposite to the first face, and each tooth of one of the connection bands being configured to cooperate with at least one tooth of the other one of the connection bands, via the first and second fastening members, so as to form links; and - a slider provided with a first opening located at a first end, with two second openings superimposed and located at a second end opposite to the first end, with a top ramp extending from the first opening as far as one of the second openings, and with a bottom ramp extending from the first opening as far as the other one of the second openings; with the opening and closing device being configured so that, under an action of moving the slider in a predetermined so-called opening direction, the teeth that cooperate enter the slider through its first opening, separate and travel respectively along its top and bottom ramps until they emerge from the slider respectively and individually through its two superimposed second openings; with the opening and closing device being configured so that, under an action of a movement of the cursor in a so-called closing direction opposite to the predetermined opening direction, the teeth of the connection bands individually and separately enter through the two respective superimposed second openings of the slider, and respectively travel along top and bottom ramps until they start to cooperate at the first opening of the slider; and with the opening closing device being characterised in that each tooth is provided with at least one first rounded portion on its first face and distinct from the first fastening member and/or with at least one second rounded portion on its second face and distinct from the second fastening member, characterised in that the first fastening member is formed by a protuberance, hemispherical in shape, provided projecting on the first face of the tooth and the second fastening member is formed by a hemispherically shaped housing, provided as an indentation on the second face of the tooth.