Von die LDraw Website:

Description: The designer, also known as LDD, is a free virtual design studio for PC and MAC OS X created by the LEGO Group. It's targeted at kids and novice users. Once you’ve finished your model, you can order the bricks from the LEGO factory and show off your design in a online Gallery.

Though it supports the import and export of LDraw models, not all LDraw models will necessarily import successfully (and models created in LDD may not be complete when exported to LDraw format), because LDD uses its own file format .lxf and parts library, which is not aligned with LDraw's.

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