You’re not going to drop upwards of $140 on a T-square unless you need a precision instrument, but if that’s what you’re looking for, Woodpeckers is prepared to sell you one of their precision aluminum T-squares. Available in 12″, 24″, and 32″ lengths, these T-squares not only allow you to draw lines perpendicular to the edge, but by putting a pencil in one of its 1/16″-spaced holes you can drag the square along the edge to produce precisely spaced parallel layout lines.
To make their precision T-squares, Woodpeckers precisely machines an aluminum blade and secures it to a solid billet handle with six precision socket screws. They drill 1mm holes every 1/16″ on the blade plus extra holes every 1″ for marking shelf pins. They laser-engrave markings spaced every 1/32″ that are accurate to .001″, and they angle the outer edges of the blade 30 degrees to make the markings easier to read. They do all this in their US shop.
Woodpeckers’ 12″ T-square will run you $80, the 24″ T-square will run you $110, and the new 32″ T-square will run you $140. At least they throw in a fitted wooden storage case for the price.