Astronomical eyepieces are described by the diameter of the base tube or barrel and come in several standard diameters. This is not to be confused with the focal length of individual eyepieces. The eyepiece diameter for your telescope is important to know when purchasing other accessories, such as cameras, adapters and filters.
Eyepiece diameters commonly found in the market are 0.965", 1.25" and 2".
0.965" eyepieces are an older size that is still used on some smaller telescopes. They are inexpensive to make and are well-suited to simpler eyepiece optical designs that have 2 or 3 lenses. For example, Nipon 300x70 refractor is fitted with these eyepieces.
1.25" is widely considered to be the standard size for eyepiece barrels today. There are many different designs and focal lengths available from many manufacturers. Almost all accessories that fit into focusers are available in the 1.25" size. Most Nipon telescopes use 1.25 inch eyepieces.
The largest eyepiece size commonly encountered is the 2 inch barrel diameter eyepiece. The larger optics in these eyepieces provides wide, unobstructed fields of view. They are heavier and more expensive than standard 1.25 inch eyepieces and are best used on larger-aperture telescopes which have larger beams of light from the optical system.
Eyepiece adapters are available, enabling a particular telescope to use different diameter eyepieces. For example, you can use 1.25" eyepieces on telescopes with 0.965" eyepiece holder, with a 0.965" to 1.25" eyepiece adapter.