Reach For A Stanley (July 2010)
Stanley Extension Rules:
A Handy Rule That Saves Time and Reduces Mistakes
How many times have you made an inside measurement only to find that you added the tape rule extra wrong and cut the stock short?
Well, the “old timers” had a solution for that called the extension rule.
The extension rule is made up of two over-lapping rules.
The rules are set up to slide by each other and can be locked in place at any point from the shortest to the longest lengths.
The scales on one rule start at the full length of the other such that the extended length can be directly read from the rule.
Originally the extension rule was used for measuring length great then the standard two foot folding rule.
As the zig zag and steel tape rules gained widespread acceptance the extension rule went out of favor. However it is such a handy tool that it never completely died out and Stanley continued to offer them into the late 1950s.
Stanley offered the extension rules in five sizes from two to twelve feet.
The smallest, No. 240 was adjustable from 2 to 4 feet, next the No. 360 from 3 to 6 feet, the No. 480 from 4 to 8 feet, the No. 510 for 5 to 10 feet.
The longest was the No. 612 for 6 to 12 feet.
While extension rules of all sizes are found from time to time with a hook on one end the only Stanley-made extension with a hook was the No. 510 H.
With the hook the 510 H was great for cutting long stock and likely became the replacement for the 10-foot pole used in earlier years.
I have found the extension rules to be great for two types of measurements.
The first is when making an inside measure such as the length of a base molding between two verticals; for this the rule is indispensable.
I set the rule between the two verticals and slide it out tight to the required width, locking the screw sets the two sections at the exact length. I can then cut the stock just a bit longer and scribe it into place.
The great thing about using the extension rule for this type of work it that you never have to add up numbers or even, for that matter, read the rule. Just set the rule, transfer the length and cut.
The second use is for making repetitive cuts.
One of the most common examples of this type of work is in cutting wall studs.
I would make a pattern for studs if I had more than a handful to cut but many times I have had to cut on a few pieces of stock to a particular length and the extension stick works very well and can save mistakes.
When cutting door casings, I have used the rule to set the length and then check it on all four sides before roughing out the stock. Just saves time and not having to read or remember measurements saves mistakes. 
Like many tools having one is the beginning to finding uses.
When working alone squaring up case frames by measuring diagonals gets to be difficult when the lengths exceed the spread of your arms. The extension makes easy work of this project.
Just set it tight to one diagonal and swing it around to check the other. You can even “eye in” the approximate amount of adjustment needed by moving the stick in or out half the amount of the indicated error. Tap the work to the new rule setting and check the other diagonal, when the two diagonal are equal the frame is square.
Stanley extensions were made for a long time and are reasonably available on the old tool market.
Many will be encountered with worn numbers but this is not a big problem, as often the length is not read. The only uncommon rule is the No. 510 H with the hook. As with all tools, prices vary with condition. A like new No. 240 will run $35 to $70 and a mint No. 510 H will weigh in at around $100. The others will fall in between with the longer rules tending to bring a bit more. Well-used and worn rules will sell for considerably less.