When using any handwritten index to local land, court, or probate records for the first time, take some time to familiarize yourself with how it is set up and organized. Indexes can vary from one office to another and the indexing scheme that was used in one location can vary from what’s used in another.
Assuming they are all the same can cause you to overlook records. This index from Clinton County, New York, indexed records by the name of the grantor and grantee, but the last names were not just broken up by the initial letter of the first name, they were broken up in to subsections based upon the first and second letters of the last name. A hurried researcher, not familiar with the index might overlook references needed.
Another good exercise is to pick one record at random in the record books and then see if it can be found in the index.