Another factor that affects image quality is Convergence.

Simply stated, convergence is the ability of the three electron beams (Red, Blue and Green) to line up and strike their respective phosphor triads and each is illuminated, to a greater or lesser extent to form white.  If the electron beams do not line up correctly, you have a slight tinge or halo around the line or character being generated by the video card in the computer.  More noticeable is the red,  and to a lesser degree the blue. Green is generally not seen as both the Red and the Blue are aligned or converged onto the Green.

Both types of CRT's are affected by this, and both types of CRT's are aligned by the CRT manufactures at the time of manufacturing. The CRT is mounted into a fixture, proper video and voltages are applied, and the tube is statically aligned. Static alignment is achieved by magnets mounted on the neck of the CRT to help deflect the electron beam. The center of the face of the CRT is the main focus of the alignment , then to a lesser degree the immediate sides and upper and lower portions,  and finally the corners.  The flatter the CRT, the easier it is to achieve alignment.

Monitors utilizing Aperture Grill CRT's more generally also provide another mode or option for convergence alignment.  This is called Dynamic Convergence, and is achieved through manipulation of the electron beam deflection through the use of coils mounted to and within the yoke (the main deflector if you will). This allows for a more precise control of deflection especially, however not limited to, the corner areas of the CRT.

Aperture Grill CRT's also are flat on the vertical plane, such as the Mitsubishi "Diamond Tron" and the Sony "Trinitron".  If you were to place a straight edge going top to bottom, you would find the CRT to be flat. 

Now as technology has progressed,  we now have totally flat CRT's, such as the Mitsubishi "Natural Flat" and the Sony "Wega". If you were to place a straight edge in either the horizontal or vertical planes,  the CRT would be found to be flat.

Currently these are the only manufacturers of CRT's that utilize the Aperture Grill technology.  If you see a aperture grill CRT in a monitor not made by either of these two manufacturers, the manufacturer of the monitor designed a chassis to support the aperture grill CRT.