When you first hear the term “T markings” many things come to mind. On the AR-15 platform, t markings are a simple why to remember where you placed an accessory or an optic along the picatinny rail along the top of your upper, along the top, left, right or bottom of your rails or handguards on your forend.
The t markings simply number every other slot along the rail section. There are two variations of t markings, just plain old t marked which is the t markings with no color added. The other is white t marked. These t markings are either white from laser etching or they can be white from paint being applied into the t marks themselves.
If your rifle has a quad rail or a “set or rails” instead of the basic plastic handguards, these too are often t marked. The numbering convention picks up where your upper receiver left off. The right side of your rails will be marked with an R then the numbers. The left side will be marked with an L then the numbers. And the bottom is marked similarly, B then the numbers.
Left or right is determined when shouldering the weapon.