The full trading range for each session is represented by the upper and lower extensions from the real body. These are called the upper tail and lower tail (also called shadows or wicks).
The complete lack of tail has significance in many candlestick patterns.
An exceptionally long tail also reveals failed momentum by buyers (long upper tail) or sellers (long lower tail), signaling a potential reversal of the existing trend.
When the open and the close are at the extreme high or low of the price movement for that time period, then there will not be any tails.
When the opening and closing price are identical or very close, the real body is replaced by a horizontal line, forming a doji candlestick.