The most basic reference point is the Standard Hydrogen Electrode, often called the SHE. Scientists arbitrarily assigned the SHE a potential of exactly zero volts. This zero value only holds true under strict standard thermodynamic conditions.
However, the SHE is quite difficult to set up and maintain safely in a normal lab. It requires pumping highly flammable hydrogen gas through a strongly acidic solution. Because of these physical dangers, chemists usually rely on secondary standard electrodes for daily work.
The silver-silver chloride electrode is the most common practical choice today. Another popular historical option is the saturated calomel electrode. These secondary reference electrodes contain solid and liquid chemicals that are much easier to handle.
They maintain a very constant electrical voltage over long periods of time. This steady voltage allows analytical chemists to accurately measure the unknown potential of a working electrode. These reference tools are absolutely essential for routine analytical chemistry tasks.
A common student misconception is that all standard electrodes must have a zero voltage. Only the primary hydrogen electrode has a zero voltage by strict definition. Secondary electrodes have specific, well-known voltages that are definitely not zero.