Primary teeth are also called baby teeth, milk teeth, or first teeth. Baby teeth are very important as place holders for permanent teeth. There a couple simple rules that usually apply to the eruption of baby teeth:
20 teeth are the usual number (10 top, 10 bottom – same as fingers and toes)
lower teeth usually erupt before upper teeth
girls teeth usually erupt before boys teeth of the same age
teeth usually erupt in pairs (left / right)
Permanent teeth usually start to erupt about first grade. A special note here is that often the permanent first molar, or six-year molar, erupts before the permanent front tooth. Additionally, the first molar erupts behind the last baby tooth and does not replace a baby tooth as occurs for front teeth. Often lower front teeth come in behind, on the tongue side, and give the appearance for awhile (can be many months) as if there are two rows of teeth.