Which tool is used to check if the casing is straight and plumb?

The tool specifically used to check if the casing is straight and plumb is the spider bob. This tool functions by utilizing gravity to indicate vertical alignment. It consists of a bob suspended from a string, allowing it to hang freely and indicate a true vertical line. When checking casing, ensuring that it is plumb is essential for both aesthetic and functional purposes, as misalignment can lead to improper door operation or structural issues.

While a level, plumb line, and measuring tape are all useful tools in construction and carpentry, the spider bob is particularly effective for determining plumb because of its simplicity and accuracy in revealing vertical alignment over longer distances, especially in areas where traditional levels might be cumbersome or impractical to use. Thus, it stands out as the most suitable choice for checking the verticality of casing.