Concept:
A try square is a marking and testing tool used in carpentry and fitting work.
Step 1: It is used to check whether two surfaces are at right angles.
Step 2: It can also be used for marking right-angle lines on a workpiece.
Step 3: It helps in marking parallel lines with proper reference.
Step 4: It can also assist in checking flatness and squareness of surfaces.
Step 5: Hence, the correct answer is try square.
\[
\boxed{\text{Try square}}
\]