bold lowercase (vector)
Say it “vector x”
Convention: bold lowercase is a vector, bold/capital is a matrix, plain italic is a scalar.
Example
In code
x = np.array([1., 2., 3.])x = torch.tensor([1., 2., 3.])Where you’ll see it in ML
Telling inputs (𝐱), weight matrices (W) and single values (b) apart at a glance.
Don’t confuse it with
- capital letter (matrix) Capital letters usually mean a matrix. In ML, X is often the whole dataset: one row per example.
- subscripts & superscript indices xᵢ is the i-th entry of a vector; x_{ij} is row i, column j of a matrix; x⁽ⁱ⁾ (in parentheses) usually means the i-th training example — not a power.