HP 54710A User's Reference Manual page 173

Marker Menu
Manual
X1 Position and X2 Position
X1 Position moves the X1 marker horizontally, and X2 Position moves the X2
marker horizontally. Use the knob, arrow keys, or keypad to change the
marker position.
You can make timing measurements with the X markers on the signal. The
difference between the marker's positions is the timing measurement. The
position of each marker is displayed, in the same color as the markers, near
the bottom of the display. Delta X is X2 minus X1. So if X1 is more positive
than X2, delta X will be a negative number, which can result in negative time
interval measurements. Also notice the 1/delta X value. If you are measuring
the period of a signal with the X markers, then 1/delta X is the frequency of
the signal.
You can also make a channel-to-channel skew measurement by placing the
X1 marker on one channel and the X2 marker on another channel.
Y1 Position and Y2 Position
Y1 Position moves the Y1 marker vertically, and Y2 Position moves the Y2
marker vertically. Use the knob, arrow keys, or keypad to change the marker
position. You can make voltage measurements on the signal with the Y
markers. The difference between the markers is the voltage measurement.
The position of each marker is displayed, in the same color as the markers,
near the bottom of the display. Delta Y is Y2 minus Y1. So if Y1 is more
positive than Y2, delta Y will be a negative number.
11–4