A mechanical drawing is a contract: it says exactly what to make and how good it has to be. Once you know where to look, any drawing decodes the same way.
Views and projection
Most drawings use third-angle projection (the symbol is in the title block): the top view sits above the front, the right view to the right. Section views (hatched) cut through the part to show internal features; detail views blow up a small area. Hidden edges are dashed.
The title block
Bottom-right: part number, revision, material, finish, scale, units, and the default tolerance block that applies to any dimension without its own tolerance. Always read the default tolerances first — they silently govern most of the part. General tolerances often reference ISO 2768.
Dimensions and tolerances
A dimension with ±, a limit pair, or a fit code (see choosing a fit) tells you how much variation is allowed. Basic dimensions (boxed) are exact theoretical values used with GD&T. Surface-finish symbols (a check-mark with an Ra number) call out roughness where it matters — see the surface-finish guide.
GD&T callouts
Feature control frames (a symbol, a tolerance, and datum letters in a boxed row) control form, orientation and location beyond what ± can. Start with the GD&T guide and the symbol reference. Read the datums to understand how the part is held and measured — that's usually the key to the whole drawing.