If you have ever looked closely at your tape measure, you may have noticed small black diamonds printed between the 19 and 20-inch marks. These markings appear on virtually every tape measure, yet their purpose remains a mystery to many. Over the years, curious users have taken to forums like Reddit and Quora to ask about them.
What Are the Black Diamonds?
On the subreddit dedicated to tools, one user asked: "What is this black diamond on my tape measure, appearing a fraction of an inch after 19 inches and multiples thereafter?" Another user replied: "The diamond-shaped marks on a measuring tape are called truss marks or black diamond markings."
These diamonds are spaced every 19.2 inches. According to the Reddit explanation, they are primarily used in construction for laying out floor joists, roof trusses, or wall studs evenly across an 8-foot span. The reason for 19.2 inches is mathematical: an 8-foot section equals 96 inches, and 96 divided by 5 equals 19.2 inches. This spacing allows for five supports per 8 feet, which can reduce material while still meeting some structural requirements.
Where to Find the Diamonds
The diamond markings typically appear at 19.2 inches, 38.4 inches, 57.6 inches, and 76.8 inches. While most residential framing still commonly uses 16-inch on center (16" O.C.) or 24-inch on center (24" O.C.) layouts, the diamond marks help when using the 19.2-inch layout system.
Historical Origins
Another Reddit user added historical context: "Diamond layout was invented based on the engineering allowed by using wooden I joists. These new joists were more expensive than traditional solid sawn lumber so they were sold with diamond layout on the idea that requiring one less joist per 8’ sheet helped offset some of the cost when switching to their product."
Wooden I joists were pioneered by Trus Joist Inc. in 1969, which is why many builders refer to all such products as 'TJIs,' similar to how many refer to any tissue as a Kleenex.
Other Tape Measure Features
In a related TikTok video from Tool Talk TV, a DIY expert explained why tape measures have a metal tip that moves slightly. He urged people to "get rid" of their measuring tapes if the metal end doesn't move, stating: "All the ends on the end of the blade of these tape measures move. This is by design. So whether you're measuring against something, or measuring away from something, that small movement in the tip allows you to get true zero."
