In interval notation round brackets (parentheses) are used for strictly less or greater than (that is, < or >). Square brackets are used for ≤ or ≥. The former is an exclusive interval while the latter is an inclusive interval. Infinity or negative infinity is always exclusive (parenthesis) when part of an interval.
Example: 1<x<3 is represented by the interval (1,3) while 1<x≤3 is represented by (1,3].
-∞<x<∞ is a boundless interval represented by (-∞,∞).