According to an example, an apparatus may include a memory that may store instructions to cause a processor to generate, for each part to be fabricated in a build envelope of a 3D fabricating device, first level descriptions and second level descriptions for the part. The processor may determine, using the first level descriptions, whether there is an arrangement that results in the parts jointly fitting within the build envelope while providing certain thermal decoupling spaces between the parts. In response to a determination that the arrangement using the first level descriptions for the parts has not been determined, the processor may determine, using the second level descriptions, whether there is an arrangement that results in the parts jointly fitting within the build envelope while providing the certain thermal decoupling spaces.