The number of ribs found on the hilt is used as an important point of identification for many of the US brass hilted bayonets found within this site.  Unfortunately, there is some confusion as to what counts as a rib.  

To count as a rib, there must be a groove on either side forming a raised section in between.  The point of confusion, comes from the hilt section between the cross guard and the first groove, as can be seen in the picture below.  Since there is no groove between the cross guard and this section, it does not count as a rib.  This leaves 16 ribs on the pictured bayonet. 

Another way to count the ribs is to count the number of grooves, 17 in this example.  The number of ribs always equals the number of grooves minus one.