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Introduction, navigation & woodTimber, lumber. The hard fibrous lignified substance under the bark of trees sizes

This bridgeA structure that allows people or vehicles to cross an obstacle such as a river or pond or garden. was constructed entirely on the back lawn, showing that you do not always need an elaborate workshop or shed to undertake such a project.

LumberAny of the framing wood. sizes

The measurements throughout this project are given in both MetricMillimeter measurements. (mmAbbreviation for millimeter which is a metric unit of length equal to one thousandth of a meter. 25.4 mm equals one inch.) and Standard/Imperial inches.
The metric measurements (mm) are given first followed by the imperialStandard. Feet and inch measurements. measurements (inch) in brackets ().
There are only two stock sizes required for this project, both being common stock sizes.
They are…
90mm x 45mm (1 1/2″ x 3 1/2″) and 90mm x 19mm (3/4″ x 3 1/2″).

90mm x 45mm means wood that is 90 millimeters wide by 45 millimeters thick.
That is called the actual sizeThe finished (dressed) size as opposed to the nominal size of a piece of wood.. The nominal sizeThe rough-sawn size of a piece of lumber. Before the lumber is surfaced, planed or dressed. The nominal size is usually greater than the actual dimension. e.g. 100x50 (2 x 4) actually equals 90x45 (1 1/2" x 3 1/2"). of the wood relates to its size prior to being dressedSurfaced; planed; smooth; even surface; gauged. (planed, surfacedSee Dressed., smoothed). For example, wood that is 100mm x 50mm (nominal size), becomes 90mm x 45mm (actual size) once it has been dressed (planed, surfaced, smoothed).

And the equivalent in standard…
1 1/2″ x 3 1/2″ means wood that is 1 1/2 (one and a half) inches thick by 3 1/2 (three and a half) inches wide.
That is called the actual size. The nominal size of the wood relates to its size prior to being dressed (planed, surfaced, smoothed). For example, wood that is 2″ x 4″ (nominal size), becomes 1 1/2″ x 3 1/2″ (actual size) once it has been dressed (planed, surfaced, smoothed).

Note: The 90mm x 19mm (3/4″ x 3 1/2″) boards used for making the arches must be premium and knotThe portion of a branch of a tree that appears on the edge or face of a piece of lumber. Usually a circular mark on a board or piece of lumber showing where the branch was. free. If there are any deformities in the boards, then they may snap when bending pressure is applied.