Throughout the race, drivers may make pit stops to change tyres and repair damage (from 1994 to 2009 inclusive, they could also refuel). Different teams and drivers employ different pit stop strategies in order to maximise their car's potential. Three dry tyre compounds, with different durability and adhesion characteristics, are available to drivers. Over the course of a race, drivers must use two of the three available compounds. The different compounds have different levels of performance and choosing when to use which compound is a key tactical decision to make. Different tyres have different colours on their sidewalls; this allows spectators to understand the strategies. Under wet conditions, drivers may switch to one of two specialised wet weather tyres with additional grooves (one "intermediate", for mild wet conditions, such as after recent rain, one "full wet", for racing in or immediately after rain). A driver must make at least one stop to use two tyre compounds; up to three stops are typically made, although further stops may be necessary to fix damage or if weather conditions change. If rain tyres are used, drivers are no longer obliged to use both types of dry tyres.