Definition
Compose shapes
Build one composite shape by fitting at least two component shapes together.
What it means
Treat the smaller shapes as pieces. Move them until selected edges meet, then look at the new outside outline. A piece is still the same shape after a slide, turn, or flip. When the goal is a filled shape, a gap or overlap means the pieces have not made that intended composite.
Remember it
Fit the pieces edge to edge; read the new outside outline.
Example
Use two matching right triangles whose perpendicular sides are 6 cm and 3 cm. Turn one triangle half a turn and place the two sloping sides together. Their outside boundary is a 6 cm by 3 cm rectangle.
Watch for
Touching only at a corner does not fill a larger two-dimensional region. Unwanted gaps or overlaps change the intended outline or area, even when the same pieces are present.
Common mix-up
Composing builds a whole from pieces. Decomposing goes the other way: it identifies or separates the smaller shapes inside a whole.
Exact meaning
To compose shapes is to place two or more shapes together so their exact boundaries make one composite shape. Pieces may be slid, turned, or flipped without changing their defining properties; the intended composite has no unwanted gaps or overlaps.