In cases where many parallel lines have to be drawn, of lengths beyond
the capabilities of ordinary set-squares, and in directions other than
square with or parallel to the working edge of the drawing board it is
convenient to have for use an _adjustable_ bladed tee-square, or one
whose blade can be set at any desired angle. The blade of such a square
should be tapered as in illustration, but shaped at its wide end as
shown, and having a stock wide enough to allow for the surface required
in the washers of the fittings necessary to make the blade adjustable.
These fittings, though requiring to be well made and neatly finished,
are not expensive or difficult to make, as they consist merely of two
washers, a square-necked bolt, and a fly nut.
The tee-square, as shown, has four parts: 1, blade; 2, fixed head; 3,
shifting head; 4, swivel. The head is held firmly by the left hand to
the left edge of the drawing board, and the blade serves as a
straight-edge for horizontal lines that may extend the whole length of
the paper. It can be used for either horizontal, or, by reversing to the
bottom of the board, for vertical lines; and, by turning it over, so
that the shifting head is against the edge of the drawing board instead
of the fixed head, lines at different angles may be drawn. The length of
the blade should be the length of the drawing board; if it is shorter,
inconvenience will be experienced when lines the whole length of the
board are wanted.