Attachment unit

An attachment unit for attaching a second device to a first device, comprising:

• a first attachment member attachable to the first device;
• and
• a second attachment member attachable to the second device, wherein
• each of the first attachment member and the second attachment member has first to Nth engagement sections arranged in the stated order, N being an integer of at least 2,
• the first to Nth engagement sections of the first attachment member respectively have the same shape as the first to Nth engagement sections of the second attachment member,
• the nth engagement section of the first attachment member is engageable with the (N−n+1)th engagement section of the second attachment member, n being an integer of at least 1 and no greater than N, and
• at least one of the first device and the second device is an image forming apparatus having an image formation function of forming images on sheets or an extension device having a function of extending the image formation function.