Telescope eyepieces are available in different barrel sizes or formats. This "eyepiece format" is a measurement of the diameter of the barrel size that drops into the telescope's focuser. There are 3 different sizes of eyepieces. 1.25 Inch - 2 Inch - and 0.965 Inch
The most common format of telescope eyepieces is the 1-1/4 inch format. You could almost call this the "standard" for eyepiece size. Nearly all brands of telescopes use this common size.
Many telescopes have the option of using the larger 2 Inch format eyepieces. 2 Inch format eyepieces will give you much larger fields of view. Some telescopes have focusers that are ready for 2 Inch eyepieces and some will require a special diagonal to convert to the 2-inch format.
The last format is the 0.965 inch format. These eyepieces are common on inexpensive department store telescopes. If you have a telescope that will only accept the 0.965" format eyepieces you can purchase adapters that will convert the telescope to accept the more standard 1-1/4 inch format.
Eyepiece and telescope brands do not need to match. If you have a telescope that is no longer manufactured and you are looking for replacement eyepieces, you do not need to purchase the same brand. You do however need to purchase the same format.