Parameters describing the X- and Y-offsets of a frame are always measured in pixels. For a normal, non-child frame they specify the frame’s outer position (see Frame Geometry) relative to its display’s origin. For a child frame (see Child Frames) they specify the frame’s outer position relative to the native position of the frame’s parent frame. (Note that none of these parameters is meaningful on TTY frames.)
leftThe position, in pixels, of the left outer edge of the frame with respect to the left edge of the frame’s display or parent frame. It can be specified in one of the following ways.
A positive integer always relates the left edge of the frame to the left edge of its display or parent frame. A negative integer relates the right frame edge to the right edge of the display or parent frame.
(+ pos)This specifies the position of the left frame edge relative to the left edge of its display or parent frame. The integer pos may be positive or negative; a negative value specifies a position outside the screen or parent frame or on a monitor other than the primary one (for multi-monitor displays).
(- pos)This specifies the position of the right frame edge relative to the right edge of the display or parent frame. The integer pos may be positive or negative; a negative value specifies a position outside the screen or parent frame or on a monitor other than the primary one (for multi-monitor displays).
A floating-point value in the range 0.0 to 1.0 specifies the left edge’s offset via the left position ratio of the frame—the ratio of the left edge of its outer frame to the width of the frame’s workarea (see Multiple Terminals) or its parent’s native frame (see Child Frames) minus the width of the outer frame. Thus, a left position ratio of 0.0 flushes a frame to the left, a ratio of 0.5 centers it and a ratio of 1.0 flushes it to the right of its display or parent frame. Similarly, the top position ratio of a frame is the ratio of the frame’s top position to the height of its workarea or parent frame minus the height of the frame.
Emacs will try to keep the position ratios of a child frame unaltered if
that frame has a non-nil keep-ratio parameter
(see Frame Interaction Parameters) and its parent frame is resized.
Since the outer size of a frame (see Frame Geometry) is usually unavailable before a frame has been made visible, it is generally not advisable to use floating-point values when creating decorated frames. Floating-point values are more suited for ensuring that an (undecorated) child frame is positioned nicely within the area of its parent frame.
Some window managers ignore program-specified positions. If you want to
be sure the position you specify is not ignored, specify a
non-nil value for the user-position parameter as in the
following example:
(modify-frame-parameters nil '((user-position . t) (left . (+ -4))))
In general, it is not a good idea to position a frame relative to the right or bottom edge of its display. Positioning the initial or a new frame is either not accurate (because the size of the outer frame is not yet fully known before the frame has been made visible) or will cause additional flicker (if the frame has to be repositioned after becoming visible).
Note also, that positions specified relative to the right/bottom edge
of a display, workarea or parent frame as well as floating-point offsets
are stored internally as integer offsets relative to the left/top edge
of the display, workarea or parent frame edge. They are also returned
as such by functions like frame-parameters and restored as such
by the desktop saving routines.
topThe screen position of the top (or bottom) edge, in pixels, with respect
to the top (or bottom) edge of the display or parent frame. It works
just like left, except vertically instead of horizontally.
icon-leftThe screen position of the left edge of the frame’s icon, in pixels,
counting from the left edge of the screen. This takes effect when the
frame is iconified, if the window manager supports this feature. If
you specify a value for this parameter, then you must also specify a
value for icon-top and vice versa.
icon-topThe screen position of the top edge of the frame’s icon, in pixels, counting from the top edge of the screen. This takes effect when the frame is iconified, if the window manager supports this feature.
user-positionWhen you create a frame and specify its screen position with the
left and top parameters, use this parameter to say whether
the specified position was user-specified (explicitly requested in some
way by a human user) or merely program-specified (chosen by a program).
A non-nil value says the position was user-specified.
Window managers generally heed user-specified positions, and some heed
program-specified positions too. But many ignore program-specified
positions, placing the window in a default fashion or letting the user
place it with the mouse. Some window managers, including twm,
let the user specify whether to obey program-specified positions or
ignore them.
When you call make-frame, you should specify a non-nil
value for this parameter if the values of the left and top
parameters represent the user’s stated preference; otherwise, use
nil.
z-groupThis parameter specifies a relative position of the frame’s window-system window in the stacking (Z-) order of the frame’s display.
If this is above, the window-system will display the window
that corresponds to the frame above all other window-system windows
that do not have the above property set. If this is
nil, the frame’s window is displayed below all windows that
have the above property set and above all windows that have the
below property set. If this is below, the frame’s
window is displayed below all windows that do not have the
below property set.
To position the frame above or below a specific other frame use the
function frame-restack (see Raising, Lowering and Restacking Frames).