Starrett started his company and one of the first products was a combination square patented May 6, 1879 (#215024). He basically cut the earlier Chaplin design in two. This gave one square edge and one 45-degree edge. He had to come up with a way to hold the rule though. So he put a groove down the length of the rule and made a "clamp hook" to hole the rule solidly. He also put holes into the rule for marking parallel lines and a scratch pin to mark with. This first patent shows a pin retaining the clamp hook to prevent it from falling out or rotating. This pin was soon replaced with different designs. These different designs were later used by most copies. The designs mostly change the clamp hook, adding small protrusions to hold alignment. A later Starrett design mad the clamp hook shaft with flats that a matching washer holds rotation. One additional feature is a spirit level. Below is a picture of two common sizes, the 12" and 6". Note the rulers have different widths and thicknesses. Other rules are also made in longer lengths. Also in this picture are the center squares Starrett patented June 29, 1880 (#229,283) for the two sizes. This design is the one that ultimately became the standard that others followed with minor changes.
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The next Starrett patent was for a protractor head, that he called a "beveling instrument". Patented Aug. 7, 1883 (#282583). This design used the clamp hook and allows a 0 to 90 degree angle with adjustable 90 stop. On this particular example (unmarked) it requires a slightly different thickness of the rule. (Maybe a manufacturing issue?) It has a spirit level. It folds up for compact storage. There is no scratch pin.
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Starrett came up with a different protractor head design and was patented Dec. 4, 1883 (#289463). This design had a single scale that goes from 90-0-90. This protractor is what is referred to as "non-reversable". That means it cannot be flipped over and used for a complementary angle. You must adjust to the different angle. This particular patent does not show a spirit level. This was addressed with a special insert that takes the place of the rule and has the level attached.
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A later protractor added a spirit level and another design has a split base the rule extends through, allowing a reversable use. This design has a dual scale but can only be seen from one side.
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The Starrett line of combination squares is still made today, and they are probably the most desired to obtain. There are currently six different protractor heads available along with many different rules. The options include either smooth or wrinkled finishes and in hardened or not treatment. They can be expensive. There are other makers that produce quality that matches the Starrett.
What is desired? Well, the best rulers are either hardened or tempered. The difference is that the hardened can break if levered (like a file could be broke) The tempered takes away some of the hardness but is still harder than an untreaded blade. The heads can also be hardened in some cases. In my experience, the hardened and tempered pieces do not rust as easily or as deep as the untreated. The reversable models are more convenient.
Part 3 will cover Brown & Sharpe
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