HP 54710A User's Reference Manual page 175

Marker Menu
Waveform
+ Position and X Position
The + marker is controlled by the + position and the X marker is controlled
by the X position. Use the knob, arrow keys, or keypad to position the
markers on the signal.
The marker position readouts are displayed near the bottom of the display.
Each waveform marker has a Y position and an X position. Vertical
measurements are made with the Y positions, and delta Y is the difference
between the Y positions. Timing measurements are made with the X
positions, and delta X is the difference between the X positions. Also, notice
1/delta X. If you are using the markers to measure the period of a signal,
then 1/delta X is the frequency of that signal.
See Also "Making Accurate Time Interval Measurements," in chapter 13, for more
information on placing markers off the display.
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