• Direct Modeling: Also known as history-free modeling. Allows users to simply push, pull, and drag geometry without worrying about a complex feature tree. Ideal for rapid conceptual design.
• Parametric Modeling: Also known as history-based modeling. Geometry is driven by specific dimensions (parameters) and constraints. Changing a dimension automatically updates the entire 3D model and linked 2D drawings. This is the foundation of modern mechanical CAD (e.g., SolidWorks, Inventor).

When a command prompts for a point, you have three primary ways to define it mathematically:

• Absolute Coordinates $(X, Y)$: Points are defined by their exact distance from the fixed Origin $(0,0)$. This is crucial for establishing property boundaries (datums) from a surveyor's coordinates.
• Relative Coordinates $(@\Delta X, \Delta Y)$: Points are defined relative to the last point entered. The @ symbol (or dynamic input mode) tells the software "from where I am currently located, move this much X and this much Y." This is the most common method for drawing a line of a specific length and direction.
• Polar Coordinates $(@Length < Angle)$: Points are defined by a distance (length) and an angle relative to the last point. Angles are conventionally measured counter-clockwise from the positive X-axis (East is 0°, North is 90°, West is 180°, South is 270°). For example, @50<45 draws a line 50 units long at a 45-degree upward-right angle.

• Model Space: The infinite, 3D void where you draw the actual object, building, or site. You always draw at FULL SCALE (1:1) in Model Space. If a building is 100 meters long, you draw a line 100 units long. You do not scale the drawing down mathematically while drafting.
• Paper Space (Layouts): The 2D environment representing the physical sheet of paper (e.g., an A1 or ANSI D sheet) that will be printed. Here, you place your title block and create "Viewports."
• Viewports: Think of a viewport as a "window" cut into the Paper Space sheet that looks back into the Model Space. You assign a specific Scale to the viewport itself (e.g., 1:100). The software automatically shrinks the 1:1 Model Space drawing to fit perfectly into the viewport at the specified scale.

• LINE (L): Creates individual, independent straight line segments.
• POLYLINE (PL): Creates a connected sequence of line and arc segments that form a single, continuous object. This is critical for calculating the area/perimeter of a room or creating a closed boundary for 3D extrusion.
• CIRCLE (C): Drawn by specifying a center point and a radius (or diameter), or by defining 3 points on its circumference.
• RECTANGLE (REC): Quickly draws a closed polyline rectangle by specifying two diagonally opposite corners.
• ARC (A): Most commonly drawn by specifying a Start point, a Center point, and an End point, or 3 points along the curve.

• MOVE (M): Translates objects a specified distance and direction (often using a base point and a displacement point).
• COPY (CO): Duplicates objects, maintaining the same base point logic as Move.
• OFFSET (O): Creates concentric circles, parallel lines, or parallel curves at a specified distance. This is the most frequently used command for drawing walls of a specific thickness (e.g., offsetting a line by 150mm for a CHB wall).
• TRIM (TR): Cuts (removes) portions of objects that extend beyond a specified cutting edge or boundary line.
• EXTEND (EX): Lengthens objects until they intersect with a specified boundary edge.
• FILLET (F): Rounds the sharp intersection between two lines with an arc of a specified radius. Pro Tip: Setting the Fillet Radius to 0 instantly creates a sharp, perfectly clean 90° corner without needing to trim/extend.
• MIRROR (MI): Creates a mirrored duplicate of selected objects across a user-defined axis. Often used for symmetrical building layouts.

Layers act like transparent acetate overlays. Every object must reside on a specific, logically named layer (e.g., "A-WALL-EXT", "S-COLS"). Never draw everything on "Layer 0".

• Color / Linetype / Lineweight: Properties assigned "ByLayer" ensure consistency. Colors often dictate the final printed line thickness via Plot Style Tables (CTB files) (e.g., Red lines print thin (0.1mm), Cyan lines print thick (0.5mm)).
• On/Off (Lightbulb): Makes the layer invisible, but the objects are still loaded in memory and regenerate when zooming.
• Freeze/Thaw (Sun/Snowflake): Makes the layer invisible and unloads it from memory. Essential for improving performance in massive files (e.g., freezing the complex contour map while working on the building footprint).
• Lock/Unlock (Padlock): Objects remain visible (often slightly faded) but cannot be selected, modified, or deleted.

BIM represents the evolutionary leap from 2D/3D CAD. Rather than drawing lines representing walls, a BIM system (like Revit) constructs an intelligent, data-rich 3D digital twin of a building.

• Intelligent Objects: A door in BIM isn't just a rectangle; it knows it's a door, has material properties, a fire rating, cost data, and scheduling links.
• Parametric Change Engine: Moving a wall automatically updates the floor plan, section views, 3D renderings, and the material quantity schedules simultaneously.
• Clash Detection: Complex structural steel and MEP systems can be modeled and automatically checked for spatial conflicts (e.g., an AC duct running through a steel beam) before physical construction begins.

• .DWG (Drawing): The native, proprietary file format for AutoCAD. It is the industry standard for 2D and 3D design data, but may suffer from version incompatibility (e.g., AutoCAD 2018 cannot open a 2024 DWG).
• .DXF (Drawing Exchange Format): An open-source, universal text-based format developed by Autodesk to enable data interoperability between AutoCAD and other non-Autodesk programs (like CNC machines or laser cutters). It contains the raw geometric data but often strips out advanced features or complex layers.
• .DWT (Drawing Template): A template file containing pre-configured layers, linetypes, title blocks, and dimension styles. Starting a project from a standard DWT ensures company-wide consistency.