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Draw the window(s) shape as a polyline, then from the pallet select Create Window from Polylines...or similar.
When you draw your Window Assembly Frame System, remember that the exterior lines actually represent the exterior Frame edge while the interior lines represent the centerlines of Mullions. You can use Lines, Plines, Arc and even Circles to create the sketch.
Once you have a frame and mullion sketch, use the "WinAssemblyConvert" command, also accessed via the Openings toolbar and the Design Pull-down menu, to Select all of your linework. To finish the conversion, simply use all of the default options for completion and you should find a 3D Window Assembly standing upright from the bottom line of your linework - as illustrated to the right.
Saving the Window Assembly Style
When you Convert Linework to a Window Assembly, the Standard Window Assembly Style is used to generate a default solution for you. In addition to using the Standard Style, ADT puts you in an "Edit in Place" mode that allows further editing prior to Saving to a Window Assembly Style Name. Though this is a great option, it's is best to Save this Style now so you don't forget that you are in "Edit in Place" mode.
To Save, Select the Window Assembly, right-click on your mouse, cascade to Save changes... via the Edit in place cascading menu option on the object-specific pop-up menu - as illustrated to the right.
On the Save Changes dialog box, use the New... button and specify a name that defines how you want your Window Assembly listed in your Schedule ( if you plan on having it in your Schedule, otherwise use any name you want ).