Object Selection, Modification, and Properties
Selection
Window selection (left-to-right) takes only objects entirely inside the box; crossing selection (right-to-left) takes anything touched. Lasso behaves the same way by direction. SELECTSIMILAR extends a selection by shared properties, QSELECT builds a filtered set by object type and property, and FILTER saves complex filters for reuse. SELECTIONCYCLING lets you step through overlapping objects, and PICKADD decides whether new picks add to the set or replace it.
Modification
STRETCH is the trap: only objects caught by a crossing window are stretched, and only the vertices inside that window move. A window selection moves whole objects instead. TRIM and EXTEND respect EDGEMODE, which controls whether invisible extensions count, and can be driven by a fence. ARRAY offers rectangular, polar and path arrays, and associative arrays keep item counts and spacing editable. OFFSET, FILLET, CHAMFER and JOIN change topology; OVERKILL removes duplicate geometry before issue.
Properties
Properties are either explicit (a colour set on the object) or ByLayer. ByLayer is the maintainable choice: change the layer and every object follows. MATCHPROP copies properties from a source object; SETBYLAYER resets explicit overrides to ByLayer. Layer 0 inside a block is special: its objects display the colour and linetype of the insert layer, which is why block geometry is often drawn on layer 0.
Worked example
A wall with a door must lengthen by 500 mm. Use STRETCH with a crossing window around the end grip and the door, then type 500 in the direction of the wall. A window selection around the same objects would move them rather than stretch the wall.
Traps
Window versus crossing confusion; stretching with the wrong window so nothing moves; explicit colours defeating a layer change; PICKADD = 0 making every pick replace the previous selection.