In the post about turning stiles for my joined stools, I mentioned and illustrated using a miter square for marking the centers of the stock. Miter squares are as simple as a try square, only instead of marking lines at 90-degrees to an edge, they usually are designed to mark a line at 45-degrees to an edge.
I know of two descriptions from the seventeenth century for this tool, the first is from Joseph Moxon’s Mechanick Exercises (1678-83) :
“18 of the Miter Square. And its Use.
The Miter Square marked E, [PF: it’s really marked “R”] hath (as the Square ) an Handle marked a, one Inch thick and three Inches broad and a Tongue marked b, of about the same breadth: The Handle and the Tongue (as the Square) have both their Sides parallel to their own Sides. The Handle (as the Square) hath in the middle of its narrowest Side a Mortess in it, of an equal depth, the whole length of the Handle: Into this Mortess is fitted one end of the Tongue, but the end of the Handle is first Beveld off to make an Angle of 45 Degrees with its inside. This Tongue is (as the Square) Pin’d and Glewed into the Mortess of the Handle.
It is used for striking a Miter-line, as the Square is to strike a Square-line, by applying the inside of the Handle to the outside of the Quarter, or Batten, you are to work upon; and then by striking a Line by the side of the Tongue: For that Line shall be a Miter-line. And if upon two Battens you strike two such Lines, and Saw and Pare them just off in the Lines, when the flats of those two sawn ends are applied to one another, the out and inside of the Battens, will form themselves into the Figure of a Square. Thus Picture Frames, and Looking Glass-Frames are commonly made…”
The other source that Alexander & I have relied upon a great deal is Randle Holme’s Adademy of Armory & Blazon (1688). Holme illustrates two versions of the miter square, one like Moxon’s;
a Miter square, of a contrary form to the following, mentioned chap. 9 numb. 17 It hath an Handle (or top part) an Inch thick, and three broad, with a Tongue of the same breadth, and for length 5, 6, or more inches, according to the breadth of the Work: it is to be Glewed into the Handle by a Mortess and Pinned.
His second illustration is a different version of the miter square, but his description still follows Moxon’s quite closely:
Book III, Chapter 9, Section 1b, #17:
“a Miter square. This Square hath also an Handle and a Tongue, as that mentioned numb.15. whose use is to strike either Square or Miter Lines according as you apply the ends of it to the out-sides of the Quarter or Batten you are to work upon; By the help of this, Miter or Bevil Lines are Cut or Sawed so exact, that two being joined together it will make an Angle; thus square Frames for Pictures, Looking Glasses, and such like are comonly made. ”
I have a few in my shop. the one I use most is the shop-made one, slightly smaller than Moxon’s or Holme’s.
I have a modern version like one of Randle Holme’s, that scribes four different angles, but I have hardly used it.
Another place where these tools are applicable, in addition to marking the centers of square turning stock, is the mitered bridle joint, sometimes seen on seventeenth-century cupboard doors from England. Here’s a few views of a sample joint: