The chief "point placement aids" are the following:
GRID <F7> (when toggled on, provides simply a rectangular visual pattern of dots on the screen which shows you approximately where things are located in 2-dimensional space). Grid only appears on the X-Y plane. The grid always is located at the current elevation level. The grid can be turned on or off independently in each viewport. There are separate grids possible for Paper Space and Model Space. The grid is of little use and is rarely used in producing architectural drawings, because it is not precise enough for the typical types of drawings made.The following illustration shows the OSNAP Cursor Menu:
SNAP <F9> (when toggled on, all points will snap to an imaginary and invisible rectangular pattern of points which you can set up the spacing of). Snap points only appear on the X-Y plane. Snap can be turned on or off independently in each viewport. There are separate snap points possible for Paper Space and Model Space. The snap always is located at the current elevation level.
ORTHO <F8> (when on, will allow you to draw only horizontally and vertically)
OSNAP (invoked by means of holding the <SHIFT> key down and simultaneously clicking the right button on the mouse (or the middle button on a three button mouse or the roller on the Microsoft "Intellimouse" ). This will pop up the so-called "Cursor Menu" which contains a list of the eleven types of points which can be snapped to. For instance, if you wanted to draw a line from the center of a circle to join the intersection of two other lines, you would select LINE, and then before inputting points, at the "LINE From point" statement, hold the <SHIFT> key down and simultaneously click the right button on the mouse, pick the word CENter from the Cursor Menu with the pick button. A square will appear at the crosshair location. This is called the "aperture." Then pick the circle within whose center you wish to connect the line to (do not try to find the real center with the aperture, but pick any part of the locus of the visible circle itself within the aperture. Next, hold the <SHIFT> key down and simultaneously click the right button on the mouse again, and select the word INTersect from the Cursor Menu. The aperture will re-appear at the crosshairs. Place the aperture over the intersection of the two lines you wish to snap to, and pick it with the pick button. The other end of the line will be perfectly "snapped" to that intersection point.
The following are points which can be OSNAPped to:CENter the center point of a circle or arc -- pick on the visible part of the circle or arc to snap to it
ENDpoint the end point of a line or arc
INSert the "insertion point" of a block
INTersection the intersecton point of two lines, arcs, or circles
APParent INTersection a point located where the intersection point of two lines or arcs would exist if the non-intersecting entities were extended so that they meet
MIDpoint the centerpoint along the length of a line or arc
NEArest any point on a line, arc, or circle which is closest to the crosshairs when picked
NODe a "point" entity
PERpendicular use as the second point in a two point pick for a line; will snap the endpoint of the line perpendicular to a line or arc
QUAdrant the top, right side, bottom, or left side of a circle
TANgent use as the second point in a two point pick for a line; will snap the endpoint of the line tangent to a circle or arc
NONe no OSNAP function (this can be used when a running mode of OSNAP has been set, and you want not to snap to anything but simply pick a point without regard to whatever the OSNAP was set to)
Remember, OSNAPping can be used only in the middle of draw or edit commands.
There is also a so-called "Running Mode" of OSNAP, which will allow you to turn on an OSNAP function which will allow continuous snapping to ENDpoints, INTersections, CENters, etc. This is useful if you have several lines to draw which will all need to be OSNAPped to the same type of points. To set up a Running Mode, select "Tools" from the Pull-Down menu, then "Object Snap Settings..." then check (by picking in the squares) the OSNAP mode(s) you wish to always invoke. When this is done, the Aperture will always be present on the crosshairs after you have picked any draw command, ready to OSNAP to the point types you have pre-selected. To turn off the running mode of OSNAP, double-click the word OSNAP in the Status Line on the bottom of the screen, or select "Tools" again, then "Object Snap Settings..." again, then "Clear All" button, or simply type OSNAP <RET> <RET>.