Abstract: A sofa or armchair with adjustable sitting depth (7-8), which has a back rest (1), which can be detached and taken off and the distance of which can be changed and thus, the sitting depth can be changed, and to which belongs an adjusting and locking device (2), with which a stepless or adjustment with steps are locked. E.g. the back of a sofa is traditionally fixed and almost all adjustment possibilities have been different solutions by cushions, which however, are not always satisfying when thinking of needs of different people. With this solution people of different lengths can adjust the sitting depth of a sofa or armchair fitting to themselves, which at the same time makes the sitting more comfortable and along with that reduces likelihood of back problems. Also the adjustable back rest gives new dimensions and possibilities for furniture design and product development.