Explanation:

Introduction:
In mechanical engineering, tolerances play a crucial role in ensuring the proper fit, function, and interchangeability of parts. Tolerances are specified to define the allowable variation in a part's dimensions. The tolerance specification consists of two values, an upper deviation and a lower deviation, which define the acceptable range of dimensions.

The Letter D:
In the given tolerance specification, the letter 'D' represents the lower deviation. The lower deviation is the algebraic difference between the basic size and the minimum limit of a dimension. It indicates the amount by which the actual dimension of a part can be smaller than the basic size while still being considered acceptable.

Significance of the Lower Deviation:
The lower deviation is used to ensure that the part will fit into its intended mating part or assembly. By specifying a lower deviation, manufacturers can guarantee that the part will not be oversized and will fit properly within the specified limits. The lower deviation is typically represented by a minus sign (-) to indicate that the actual dimension is smaller than the basic size.

Example:
Let's consider an example to understand the concept better. Suppose we have a shaft with a basic size of 25 mm and a tolerance specification of 25 D6. In this case, the lower deviation (D6) would be -6 μm. This means that the actual dimension of the shaft can be as small as 24.994 mm (25 mm - 6 μm = 24.994 mm) and still be considered acceptable.

Conclusion:
In conclusion, the letter 'D' in the tolerance specification represents the lower deviation. It is used to specify the allowable amount by which the actual dimension of a part can be smaller than the basic size. By specifying a lower deviation, manufacturers ensure that the part will fit properly within the specified limits.