An article (1) for forming an aerosol. The article (1) comprise an aerosol-forming substrate (2) and a first major surface (1a) which is substantially flat. The aerosol-forming substrate (2) comprises a first region (20) having a first density and second region (21) having a second density. The first density is different to the second density. The article (1) is insertable into the heating chamber of a device for generating an aerosol.