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Plans are a set of two-dimensional diagrams or drawings used to describe a place or object, or to communicate building or fabrication instructions. Usually plans are drawn or printed on paper, but they can take the form of a digital file. Plans are often for technical purposes such as architecture, engineering, or planning. Their purpose in these disciplines is to accurately and unambiguously capture all the geometric features of a site, building, product or component. Plans can also be for presentation or orientation purposes, and as such are often less detailed versions of the former. The end goal of plans is either to portray an existing place or object, or to convey enough information to allow a builder or manufacturer to realize a design.

The term plan may casually be used to refer to a single view, sheet, or drawing in a set of plans. More accurately, plan refers to an orthographic projection looking down on the object, such as in a plan view, floor plan or bird's-eye view.

The process of producing plans, and the skill of producing them, is often referred to as technical drawing.

Plans are often prepared in a set. The set includes all the information required for the purpose of the set, and may exclude views or projections which are unneccessary. A set of plans can be on standard office-sized paper or on large sheets. It can be stapled, folded or rolled as required. A set of plans can also take the form of a digital file in a proprietary format such as DWG or an exchange file format such as DXF or PDF.

Plans are often referred to as "blueprints" or "bluelines". However, the terms are rapidly becoming an anachronism, since most copies of plans that were formerly made using a chemical-printing process that yielded graphics on blue-colored paper or, alternatively, of blue-lines on white paper, have been superseded by more modern reproduction processes that yield black or multicolour lines on white paper.

Plans are usually scale drawings, meaning that the plans are drawn at specific ratio relative to the actual size of the place or object. Various scales may be used for different drawings in a set. For example, a floor plan may be drawn at 1:50 (or 1/4"=1'-0") whereas a detailed view may be drawn at 1:25 (or 1/2"=1'-0"). Site plans are often drawn at 1:200 or 1:100.

Further information: Architect's scale and Engineer's scale

Because plans represent three-dimensional objects on a two-dimentional plane, the use of views or projections is crucial to the legibility of plans. Each projection is achieved by assuming a vantage point from which to see the place or object, and a type of projection. These projection types are:

• Orthographic projection, including:
  • Plan view or floor plan view
  • Elevation, usually a 'head-on' view of an exterior
  • Section, a cutaway view of the interior
  • Axonometric projection, including:
    • Isometric projection
    • Dimetric projection
    • Trimetric projection
• Oblique projection, and
• Perspective projection

• Blueprint
• Engineering drawing
• Floor plan
• Plat
• Technical drawing