Dear Georgia Braille,
As you state in your question, Section 18.104.22.168 states, "Locational (Secondary) Lines...tick marks on number lines and graphs...must be 1/2 inch total...1/4 inch on each side of line... For clocks, the tick marks should straddle the circumference line and extend no less than 3/16 inch and no more than 1/4 inch on either side of the circumference." Our feeling was that 1/2 inch for clock was too long. The tick marks on clocks can range from 6/16 inches to 1/2 inches on the circumference line. The smaller the clock face the shorter the tick mark.
22.214.171.124 3rd bullet, "Tick marks on the horizontal and vertical axis lines...should cross the axis lines, 1/4 inch on either side of the axis line." This was meant to imply a total of 1/2 inch for the tick marks on the lines of a Cartesian Graph. Example 10 in the TG Supplement shows where tick marks on a Cartesian Coordinate graph do extend on both sides of the X and Y axis lines.
The examples shown on 6-40 and 6-41 are the braille examples...meaning that the TG Guidelines do NOT show the print. These braille examples were meant to show the alignment and placement of the labels in relation to the vertical grid line. They were not meant to be interpreted as "tick marks." These short lines on one side of the x and y-axis lines are not tick marks, they are vertical grid lines (an extention of the grid lines).
In response to your last comment about none of the examples showing the the vertical grid lines being 1/2 inch...these examples are reduced in size in order to fit on the print page. In actuality...these vertical grid lines should be 1/4 inches.
Hope this helps, if you have more questions, let me know! Diane