This study examined the relationship and degree of predictability that the fluency of writing the alphabet from memory and the selection of allographs have on measures of fluency and accuracy of spelling in a free-writing sentence task when keyboarding. The "Test Estandarizado para la Evaluación de la Escritura con Teclado" ("Spanish Keyboarding Writing Test"; Jiménez, 2012) was used as the assessment tool. A sample of 986 children from Grades 1 through 3 were classified according to transcription skills measured by keyboard ability (poor vs. good) across the grades. Results demonstrated that fluency in writing the alphabet and selecting allographs mediated the differences in spelling between good and poor keyboarders in the free-writing task. Execution in the allograph selection task and writing alphabet from memory had different degrees of predictability in each of the groups in explaining the level of fluency and spelling in the free-writing task sentences, depending on the grade. These results suggest that early assessment of writing by means of the computer keyboard can provide clues and guidelines for intervention and training to strengthen specific skills to improve writing performance in the early primary grades in transcription skills by keyboarding.