Heads:

The basic Starrett combination square head has a single screw with witch to lock the blade. The blade can be reversed in the handle if need be. The head handle contains a spirit level and a steel scriber.

Attachments:
In addition to the basic square head you can also get three attachements for the square: The center head, and the reverseable and non-reversable protractor heads. You can always add accessories later on although it?s less expensive to do so when you purchase the basic square.

The center head is used for marking the center point of a dowel or rod. It?s easy to use, although you should always mark several lines because the dowel may not be perfectly round. It?s primarily for lathe work but I do use it occasionally. It happens to be one of those accessories that if you need it it becomes indespensible.

There are two protractor heads which can be used separately or in combinaition with the square head. This is an incredibly accurate way to measure and layout angles. What is shown is a reversible protractor head. What reversible means in this context is that the protractor forms a shoulder on each side of the rule. There is also a non-reversible protractor head which is similar except the blade of the bevel comes down on only one sides of the rule. The good part of the reversible head is that you can easily flip the square, the bad part is more of the rule is covered so it is harder to read. On the back of the protractor is the lock to hold the angle and a small spirit level which is handy because with it you can easily measure angles as an offest from level. Of course all the attachments can slide off the rule when you don?t need them. Protractor heads don't fit rules less than 12" long.