You know the old saying - how many men does it take to screw in a light bulb? Well, if you don't pay attention to the size and base of the bulb you need, something so simple as screwing in a light bulb could be trickier than you think. So, if you don't know off the top of your head exactly what a 50W MR-16 GY-5.3 12volt Halogen bulb is, then you might still need a little help. The guide below was created to explain everything you ever wanted to know (and more!) about bulbs.
The glass component of a bulb can come in a number of shapes, each coded with a letter combination.
A Series are standard, pear shaped bulbs, and are most commonly used for general applications
B & CA Series are typically used with chandeliers and are decoratively shaped as Candle flames (CA), or Bombs (B - oval shaped). These can also be referred to as Torpedo Tip - TT (see B-10) or Flame Tip - FT (See CA-9.5)
BR Series, which stands for Bulged Reflector, is similar to the R shape with the addition of a bulged end
C Series are Cone shaped bulbs, sometimes used for night lights and other small fixtures
F Series, also known as Fiesta or Flame bulbs, are similar to B series but have decorative additions of ridges and color
G Series are Globe shaped, and are commonly found on most bath vanity fixtures
MR Series, or Multifaceted Reflectors, are similar to PAR series but incorporate a multifaceted mirrored surface which creates a higher intensity light output, often used in track and spot lighting
P and PS Series are shaped similar to A series, but are used in high wattage fixtures requiring over 200 watts
PAR Series, which stands for Parabolic Aluminized Reflector, is similar to the R shape, but provides halogen lighting and is more efficient than R lamps. PAR bulbs are also weatherproof and can be used in more locations than R or BR lamps
S Series are Straight sided bulbs most commonly used in signage and specialty or indicator applications (often in vehicles)
R Series, or Reflector bulbs, which incorporate a mirrored coating on the back of the bulb to aim light forward, are most commonly used for floodlights, spotlights, and track or recessed lighting for directional illumination
T Series are Tubular bulbs, often used in picture lighting, piano lamps, and other long fixtures
The diameter of a bulb is designated as a number following the letter which denotes the shape. A bulb's width is always measured in increments as eighths of an inch, for example, a standard A-19 bulb has a maximum diameter of 19 eighths of an inch, or 2.375 inches.
Now that you've determined the size and shape of the bulb you're looking for, you'll need to find the right size base to fit the socket in your fixture. Bulb bases are measured in millimeters (as opposed to the bulb size, which is measured in eighths of an inch).
Screw Bases are noted by the letter E, which stands for Edison, followed by the size of the base in millimeters. A standard A-19 bulb has a medium E26 base, which is 26 millimeters in diameter
Bi-Pin Bases have two contact points which protrude from the base. The diameter of a Bi-Pin base is measured as the distance in millimeters between the two pins. For instance, a GY6.25 base has a distance of 6.25mm between the two pins
Bayonet (or push-twist) Bases are used with sockets with spring loaded base plates. The wiring wraps around the outer side of the base, and makes contact with the conductor plates within the inner sides of the socket. A prime example of a small bayonet base is a miniature holiday light (see slide #1 and slide #2), which slides into the socket, transferring electricity through the wiring that runs along the outer sides of the plastic base. Bayonet bases are commonly used in European fixtures.
Wedge Bases operate in a similar manner to Bayonet bases, but are constructed of glass or plastic
Philips Compact Fluorescent Lamps (CFL) PL-T Base Diagram