What Is a Basal Body Temperature (BBT) ?
Your basal body temperature is your body’s temperature at rest. To get an accurate measure, your temperature must be taken in the early morning, before getting up or moving around. (Sitting up and walking around increases your body’s temperature slightly.) if possible, while you are still "half asleep". The best reading of your basal temperature can be taken at the same time every morning when you first awake.

Charting your basal body temperature is a way to track ovulation. After ovulation occurs, most women notice a sustained increase in their basal body temperature.
Basal temperature is the lowest normal body temperature of a person immediately upon awakening in the morning.

Why Is BBT Charting Helpful?
Charting your basal temperature is helpful in determining whether and when ovulation occurs. Ovulation is the release of an egg (ovum) from the ovary and ordinarily happens only ONCE in each menstrual cycle. Conception can take place only if intercourse takes place at or near this time, during the interval of transition between low and high temperatures.
By carefully recording your basal temperature when you first awaken, you will begin to notice a pattern of temperature fluctuation in your monthly cycle. The slight increase in basal temperature is an indication of ovulation or fertility by which you can regulate a pattern on activity to either achieve conception or avoid pregnancy.

Boost your baby-making chances: chart your basal body temperature and cervical mucus

Looking for a cheap, low-tech way to pinpoint your fertile time? Experts say the best way to detect ovulation is to combine a daily reading of your basal body temperature (your temperature when you first wake up in the morning) with monitoring the texture of your cervical mucus. Chart your cycle for a few months so you can recognise your pattern and have a better chance of predicting your most fertile days.

How to do it: Start keeping track of your temperature on the first day of your period using a basal body thermometer, which shows minute changes in your temperature. A digital thermometer can also work, although it's not as accurate. Take your basal body temperature and plot it on a chart. Then check your cervical mucus. Using your fingers or toilet paper, note its consistency. You may also want to monitor its texture throughout the day. Most women will see a spike in their temperature, signalling that they've ovulated, and notice that their mucus looks and feels like raw egg white at around the same time. When the two coincide, it's baby-making time. :-)

Basal means high accuracy and all Basal thermometers have to measure to a minimum 2 decimal places, be careful as other eBayers sell 1 decimal place Thermometers as Basal which is false.